For those of you with clients...

Just a general question. As I've said elsewhere, I don't develop for clients, but I'm curious how it works with updates for you guys.

When you develop for a client does the site just remain stale (without software updates) until you get called back in? Or are you kept on some kind of retainer? Or do you check in from time to time with a suggested tweak? (And do you charge for that?)