I'm not aware of any fixed guidelines, but I now go with the following guidelines:
When Creating New Characters:
When creating new high-level characters, I pretty much stick with the "big six" items - stat boosts, magic weapon, magic armour and/or shield, ring of protection, amulet of natural armour, cloak of protection. (And for Rogues, the "skill boost" items.)
Basically, stick with the "effective but dull" items. In general, I'd go for about a third on protection, a third on stat boost, and a third on offense (which for a warrior is a weapon; for other classes it varies of course).
When Assigning Treasure:
When assigning treasure, I generally take the Wealth-by-Level number for the current level and subtract it from the WbL number for the next level. Then multiply that by the number of characters in the party, and add about 10% (for disposables).
I then divide that by 3, and place about that amount of stuff in raw 'treasure' form - big piles of gold, jewellery, artwork, etc. Any magic items that the party cannot use also go into this category. All items are placed at their sale value, under the assumption that that's how the party will use them.
The remainder is placed as items for the party to (maybe) use. And here, again, I assume that they're going to sell them, so I double the budget to get an amount to assign.
But then I assign treasure deliberately avoiding the items that the players would choose if they were picking for themselves. So, there won't be any +1 holy long swords, headbands of intellect, cloaks of resistance or the like in their haul - if they want those items, they have to buy them themselves. Instead, they get technically-less-effective but probably more interesting items - boots of desperation, a crystal mask of dread, etc.
In theory, this then gives the players an intesting choice - they can either keep the items they get (nominally more valuable, but not optimised), or they can sell them on and buy up the "powerful-but-dull" items in their place.
(I should note that for NPCs I tend not to be too exacting in assigning the modifiers - rather than calculate every +1 here and there, I essentially build them with "inherent bonuses" switched on. So, the NPCs aren't dependent on having the powerful-but-dull items that I don't want to give out as treasure.)