I use a combination of selected/placed treasures as well as random rolls (which generally get adjusted up/down based on how well I feel they played out).
In terms of resources, a few of the guys in our play group created the
Treasure Generator which is hosted over on Dragonsfoot for download: it's a compilation of all of the tables from the 1e DMG and UA, and will generate random magic items including fully-populated scrolls, the various additional random sub-types of magic items (which version of scroll of protection from elementals, which figurine of wondrous power, the 10% chance that your ring of regeneration is vampiric, the very slight chance that your scarab of protection is +2, etc., etc.).
I'm also a huge fan of the Necromancer Games/TableTop Games
Book of Treasure Tables, which is the only post-1e book that I regularly carry to games. This is a wonderful trove of excellent (although non-magical) random treasure tables, ranging in value from 1gp up through some huge figure suitable for a dragon's hoard (perhaps up to 1,000,000gp?). It's OOP now, but still one of the best D&D books ever published---well worth hunting up a copy that you feel is reasonably priced.