I'm going to try to be a little more nuts and bolts, since that's what Evenglare seems to be asking for.
For me, 4e has put me in a place where I focus almost all of my energy on planning encounters. I don't always like this, and I'm working on trying to bring back more story-oriented thinking, but the encounter-prep approach makes sense.
The first thing I prep is the monster stat blocks. I use Power2ool.com, which is the best tool I've used (Sorry Monster Builder!). I can throw together customized monsters, in batches for each encounter, add in magic items if necessary, all in a matter of minutes, then print the sheets and be done.
Second I prep the maps. My first pass on maps is just getting them into something like Maptool (rptools.net) (we use a projector for maps in my game, rather than tiles or a megamat).
If time permits, I'll spend some time with the map trying to figure out some details, terrain effects, etc, and really make the map interesting.
The day of, I prep minis (we use
action stands, which I attach to the minis with sticky tac) and initiative tracking tents with pre-rolled initiative.
The Good:
This is nice and systematic. I know for one of our six hour sessions I need to have 6 encounters prepared, with one that I can throw out towards the end of the session if we've been playing slowly. I'm a gadget nerd, and you'll have noticed that I'm using a lot of different tools that help me do it quickly. The only part of it that is totally necessary is first part, in my opinion. I could, if I was short on time, turn off the projector and draw maps the day of the session, and the action flags are nice and handy, but not critical (and could be managed during play, either by me or by a player).
Also, almost all of the prep can be done outside the game room -- I put the monsters and maps together sitting at the computer, so I can do it during a few stolen moments at work, or sitting in a coffee shop, etc.
The Bad:
This path leads to railroading, and a combat focused game. Sure, skill challenges can be mixed in with the other encounters, and I usually do that, but they tend to not get the same sort of attention (something I'm working on). But because I'm preparing encounters, and time is short, I don't usually take the time to prepare extra encounters in case the PCs make choices I don't expect, and when I have prepared encounters I tend not to be very open to ad libbing encounters based on the PCs making unexpected choices -- so I don't leave opportunities for those choices.
I try to counter that by doing a couple of things:
1. I don't usually prepare more than a session ahead -- PC choices are a lot easier to respond to if I'm not very far ahead of the PCs.
2. I have been investing a lot of time thinking about non-combat parts of the game lately -- especially elaborate skill challenges. This can be very time consuming, but it makes it possible to break up the combat slog and try some different ideas.
When I'm preparing home-brew stuff I also spend time journaling and thinking about the important NPCs in the adventure -- I write about their goals, how they respond to the meddling PCs, etc. This helps me map out the next session's encounters, and also gives me good prep and fodder for ad libbing encounters once the PCs go off the rails, and keeps me from having to dig around in my notes too much looking for what I wanted a particular NPC to say in a given scene.
It occurs to me just this minute that there's no reason I wouldn't do that with NPCs in an off-the-shelf adventure. But I've never tried it.