The game I am currently running is "core rules only," but I was spending some time looking through another supplement for some spells to introduce via a found spell book (there just aren't enough necromancy spells in the core book to make an interesting and strange mid-level necromancer's book) and I got to thinking/wondering about how others introduce new material to their games.
Do you wait for a new campaign to start up to introduce new class powers/variants, feats, etc. . ?
Do you find things in new books to recommend to players?
Do you find things to recommend to the DM to use or allow you to use?
As DM, do you make players make a case for new material or do is it anything goes as soon as it comes out?
How do you handle owning the supplemental rule books? Must everyone have a copy? Just the DM? Just the player using the new material for their character? Something else?
So what are your guidelines, plans, approaches, limitations, stories of what worked and what didn't for introducing new stuff?