I know magic items are unnecessary but

Over the life of the PCs (20 levels of adventuring), per PC in the party, I will place:

* 3 attuned items
* 3 unattuned items
* 6 consumable items (potions, scrolls, etc...)

These items are 50% likely to be randomly determined on the tables, and 50% likely to be designed by me to fit the needs of the party. Even when I randomly roll a magic item, I always add a quirk to it to make it distinct (a Rapier +1 might be able to create a spell effect once per LR, might deal the +1 damage as radiant damage, might allow the wielder to extend their reach by 5' for 1 minute 1/day, etc... while a wand of magic missiles might also grant the light cantrip (only castable on the wand), might collapse to the size of a coin, might vibrate on command, might be usable on a bonus action 1/day, etc...)

I will also place roughly:

* 750 gold per PC of treasure between levels 1 and 5,
* 7500 (more) gold between levels 6 and 10,
* 75,000 (more) between levels 11 and 16, and
* 75,000 (more) between levels 17 and 20.

750 is enough to buy about 2 uncommon items, 7500 is about enough to buy 2 rare items, 75,000 is enough to buy about 2 very rare items, and the additional 75,000 should be enough to get them 1 legendary item (maybe). PCs rarely buy this amount of stuff because I often provide other things for them to spend money on (such as land, bribes, taxes, ransom, etc...) Consumables are the only items that do not follow the price suggestions in the DMG. Buying spell scrolls (and consumables in general - I try to equate them to a spell level) is always subject to negotiation, but my general price guide is:

Cantrip: 5 GP
Level 1: 25 GP
Level 2: 50 GP
Level 3: 125 GP
Level 4: 250 GP
Level 5: 750 GP
Level 6: 2500 GP
Level 7: 5000 GP
Level 8: 12,500 GP
Level 9: 25,000 GP

I tend to give out too much, like LMoP where there are like 25 items. A lot are potions and minor scrolls, but there is still a lot compared to what I see others saying. In 5e I have been giving out +0 weapons with other powers along with +1 things. Most PCs have a magic attuned item by 3-4th level and a few minor things. The thief in the party found a set of bracers that he can pull daggers from and lets him misty step 1/rest. This was at 4th level. More rare weapons may need to wait longer and possible require a quest themselves. A player choosing a falchion over a longsword for example. I'll give out one to make the character the player wants, but may make him wait for it. I did have a local duke have one in his basement as a reward saying, "I have been trying to get rid of this piece of junk."

By the end of my last campaign (six PCs around 15th level), I think every PC had at least one attuned magic item. One or two of them had two. There was a scattering of +1 or +2 weapons. They probably had a dozen potions or so, but never used them, mostly because they forgot about them. The healing potions weren't particularly helpful. It was generally more efficient to use a spell to heal than a potion.

If it made sense that there might be a magic item in a particular treasure horde, I'd use the random treasure table to determine if any were present. But I picked the actual items myself from the tables so that the party got things they could actually use. This was mostly because they weren't likely to find anywhere to sell or barter them, based on the setting. In another campaign, I might have gone completely random. It seemed like an appropriate amount for the campaign. They didn't unbalance the game in any way.

The key is to know what is appropriate for your campaign. The game is flexible enough that if you go a bit too far you can compensate. You can up the opponent's HP, or pick opponents with particular immunities to lessen the impact of the party's magic items. That only helps so much, though. My advice is always pay attention to the effect a new magic item has on the game before considering any more. Be careful how many you give out at a time. Probably one is best, not more than two.

I have a few methods:
1) if running a module, then whatevers listed. Assuming the party finds it. If they don't search area x, don't encounter various npc/monsters, etc then they miss loot. It doesn't just magically relocate ahead of them, not even if it's plot important.

2) From an npc/item using monster. These items are chosen to be usefull to my npc/monster. How usefull they are to the players is not of concern.

3) completely random. I like rolling on random treasure tables in lairs, if the characters go shopping, etc. I tend to use my 1e DMG & a bag of scrabble tiles (to determine the "letter type" (it's a 1e thing).
I don't care what items are generated this way. I'm OK with things that radically swing the game if rolled. If I weren't, I wouldn't be rolling.

4) chosen items. Things that I want included in the game for some reason.
For ex: the Apparatus of Kwalish - I just like this thing. So it tends to show up in my games.

I've let the adventure drive how many and how often to hand out magic items. However, when converting the 3.5 adventure Red Hand of Doom, I decrease the number of magic items in many cases. I also add unique features to every item using the table in the DMG. Many items end up needing blood to operate. In fact, I got a little bored with this roll and started adding specific creature blood. So, when the party happened upon a magic item that required a drop of fresh troll blood daily, they decided to leave that item behind. They didn't want to carry a troll body part around. As it is, the warlock character sacrifices a fowl every morning to keep his magic gear running and then cooks the bird for breakfast.

I'm also experimenting with magic items that level up in power with the characters. These items are about equivalent in power to legendary items in that it is a combination of about three other magic items by the time the character is level 17. I'm not sure I like how they work out because it often makes me wonder if I should remove a magic item from the treasure description knowing that the item in the description will most likely be sold.

The formula I try to keep to is (for a party of 4):

1 useful permanent item per level (getting progressively better as time passes)
1 consumable magical item per level/player.

This seems pretty close to the D&D standard.

I do it a bit like @Ath-ketin . Magic items cannot be given out totally random, unless you want them to become the unplanned Focus of your campaign. Since i have a General rough Overall planning in my campaigns tend to give out Magic weapons (Tier1 i mean e.g. +1 weapons) to the fighter / rogue types at least, if the Party is to Encounter creatures with resistance to nonmagic damage a lot from a certain stage in the campaign, unless it is a very low Magic world.

In my games a Group gets several logistic items never without thought: wings, transportable Portals, bags of Holding / portable holes and such.
They might get a ring of featherfall quite early, this item for 1 Party member has no danger of unbalancing things. I am quite generous also with items that give a small boon to a stat or metaskill. The only thing whith which i am very careful, out of experience, is things that boost pc armor class significantly.

Depends how you want magic items to be viewed, really. If you want them to be a common treat, then random rolls are fine, drop them in any time you like. On the other hand, if you want them to be special, then it's much better to think about each one individually, and dole them out carefully over time, starting small and slowly building up (you don't want your players getting jealous because one character got a huge boost early, and they don't have anything yet...)

[MENTION=2629]jgsugden[/MENTION] #11 Very good post, and good System to spend coin on, i especially like how you exponentially increase cost by scroll Level. I do it quite similar, but i did not offer other Magic items than scrolls or potions to be bought by the Players. I got some other goodies although like compass, clocks, war machines, vessels coaches etc. for my current campaign.

A question: Do you always follow this rule, no matter what campaign? (Asuming you do not only run 1 style of campaign which makes this question obsolete)

Each session, I generate 10 treasure hoards using this site: http://www.mithrilandmages.com/utilities/5ETreasure.php

Then I pick out the most interesting items and place them as treasure throughout the session. I particularly like placing potions and other single-use items. My players level-up about every 2-3 sessions, so they're getting the most interesting picks of 20-30 rolls on the hoard generator.