- Use an established setting- this gives you all kinds of lore and locations and people and organizations to work with, and likely maps and other useful material that you can use without having to make it yourself. It may also have the benefit of grabbing your players' attention because they already have some connection to the setting. Alternatively, it may do the opposite if you have someone who hates a given setting for some reason. Think about what you need the setting to be or do.....then take a hard look at your homebrew world and see if there's a published world that largely does the same thing. If so, ask yourself that hard question....does this need all the work I'm about to put into this? Or should I just buy and read this other book that has a similar world? Or can I get what I want by taking this established setting and giving it a few tweaks? This is probably the biggest time sink.....the idea that the GM must or should craft an entire world himself. It's hard to accept that the players will likely not see a substantial difference from a homebrewed world and a published one. They may.....and there may be an idea you have that will create a truly unique setting that hits the notes you want it to. But you have to decide if the effort is necessary, or if you can achieve what you want with far less work.
This is a good idea. Starting with published stuff also gives you an idea of what you need, if you ever write your own. The fact I struggle to make sense of published settings doesn't make this bad general advice, just advice I'm unlikely to think to give.
- Use and reuse stat-blocks- create a stable of stat-blocks for characters of a specific type. Need a court wizard who counsels the duke? Need an evil wizard who is gathering a force in the nearby mountains? the same stat-block can be used for both of these and many other characters. The same can be said for many other NPCs that you'll need. You don't need to craft each and every NPC that your players will run into....it's a colossal waste of time.
This is part of why I write things on index cards. Granted, it's more monsters than NPCs, but I do re-use them. A parallel is to not worry too much about NPC stats until they matter. If you have a grand vizier sort, just have an idea until/unless you need his stats. If the players never engage, they'll never miss the work you didn't do.
- Obtain a variety of material- buy or find material that you can use in play, whether these are battle maps or dungeon tiles or what have you. Anything that will save you time from having to create maps yourself, whether regional maps, or tactical grid maps. Read as many adventures as you can, especially from alternate publishers. Even if you don't plan to actually run the adventure aspresented. See more approaches to play. Break the adventure apart into its components and see what you can use. Check out material from sources you might not normally consider, like OSR books and zines, for example.
This is good. Also, third-party products for things like monsters or items or spells, if you want to throw the occasional curveball at your players. You might need to vet with some care, but there are guidelines in the DMG that are helpful, here.
- Steal ideas shamelessly- There's nothing wrong with lifting things from other media. If you need a couple of NPCs on the fly, think of a pair of characters you know from a book or movie, and then morph them to fit your need. I had a pair of NPCs that were basically Riggs and Murtaugh from Lethal Weapon. It gave me physical descriptions I could use and also a kind of RP angle for each. All I did was write "Guild Mercs, Riggs and Murtaugh" and then pasted the relevant stat-block. I didn't have to write a description of them or character traits or anything else, just a trigger that gave me everything I needed at the table when the time came to run them. Steal frequently and indiscriminately.
This is kinda related to the previous one, but the idea of snagging ideas from other media is solid. Even if your players don't catch the references, it'll make it easy for you to remember. I have some powerful quasi-deities in my setting, one of whom is named for Dante Alighieri and another is named for Randall Flagg.
- Use player offered information- I almost hesitate to mention this because I don't want people to take it as going as far as previously mentioned in this thread. However, you can still maintain a GM focused style while taking cues and ideas from your players. Steal from them for your campaign. One of the PCs stole a relic from some dark church? Use the church. Another PC says that his adventuring uncle is the reason he's become an adventurer himself; have the uncle show up. Place these things into the world whenever you can. Replace your ideas with these if you need to, or merge them if possible.
This. Very much this. Character actions and decisions have consequences. Also, if your players start speculating around the table, take that as an opportunity to either prove them right, or to prove them wrong; either result can be fun for all concerned.
- Draw maps, leave blanks- this is advice from Dungeon World, so I know it starts to push into changing GM style, but I think it's still useful to keep it in mind. You can also think of this as "hold on loosely" to your plans. Leave some details to be decided in play. Exercise the improv muscles, and with time, you'll be comfortable with determining some things on the fly. I don't mean to leave the dark temple unmapped and unpopulated, but those couple of rooms down the side hallway? Leaving them blank saves you time up front, and gives you the flexibility to adapt to how play has actually gone at that point. If things have proven far easier than expected, you can have one of the rooms contain another encounter. If things have gone horribly for the PCs, then maybe the room is an alchemist's lab, and the PCs can find some potions that might help them a bit. I know this may not work for some folks, but it's advice that I think is good to consider.
This is part of why I don't prep more than the upcoming session. I often/usually don't know where the players are going to be at the end of the session, so it's easier to prep less. Overflow happens, and isn't bad, and stuff that gets missed can be recycled. This isn't to suggest going all "quantum ogre," but you can certainly use a monster block later in a different context, or shift parts of an idea the party never encountered.
- Examine your at table practices- do you track all HP yourself? Do you also track initiative? What do the players do during play? Think about these kinds of practices and what they mean for your game. I've played ina game where the GM tracked all damage, never actually giving numbers, but instead narrating the loss of HP for all characters, including the PCs. This created some uncertainty for the PCs and made them very cautious because they were never 100% sure how far they were from 0 HP. But this requires a lot of effort on the GM. This isn't about prep so much as easing the burden in play, but I'd include it as similar advice.
Again, this. Know yourself, and how much bandwidth you have. I don't have a problem tracking initiative, but some tables find it faster/easier if someone other than the DM does so. If there's, say, a party-allied NPC, don't be afraid to hand that off to a player to run in combat. Also decide whether you want ACs and DCs to always be secret. I usually announce ACs, at least after a couple rounds. Encourage players to roll to-hit and damage at the same time (and seriously consider doing this yourself).
- Organize- related to the above of examining table practices, do what you can to properly index things and lay them out for use at the table. As someone who likes to use multiple monster and/or NPC stat-blocks in a single encounter, it can be a pain in the ass and a big time waster to sit there and flip pages to the required stats. Now, this is more about in play, but if you organize ahead of time and develop a system for how you prepare and arrange the information you'll need in play, then it'll cut down on the amount of time that you spend on this stuff.
This is a big part of why I use my index cards for monsters and occasional NPCs. Even if I have to sort through a stack of index cards, that's still quicker than finding things across three or four books.
There are plenty more, for sure. Others likely have ways they would suggest that I haven't even thought of. It's certainly going to be different for everyone. I think the biggest thing to do is to examine your practices and decide what you want to improve, and then consider anything that may affect that. Must you home brew the setting? Does it actually enhance play for all? Can you repurpose materials for your use, whether it's a stat-block or a map or anything else? Give all these things serious consideration, and then decide what you want to do.
I'm not really thinking of things that aren't in this. There are some people who try to get the prep onto one page, so there's no flipping. I try to make it easy to read, so there's white space, so it takes more pages. Tastes will vary here. I also try to have a solid handle on what's going on and use the dice kinda like an oracle--I'm either rolling to see how much a given NPC knows, or I'm rolling to see whether he sees through a PC who's trying to snow him; makes things a little less predictable.
If you're writing your own adventures and your stuck, try an oracle. Tarot or I Ching, if you feel comfortable doing so, or something like Rory's Story Dice (which you can get as an app) if you prefer something without mystical baggage. In either case, you're not completely bound to the oracle; just use it to see if a connection you'd missed emerges, or as a starting idea from which you can diverge. This isn't quicker, necessarily, but it might be easier and/or less stressful.