Session Time-out?

Hi,
I know this is an old problem and I've googled for it, but I've not really gotten wiser ...
My problem is the following:
I want my users to log on and then edit or add data via web forms. Sofar, so good. But I want to prevent time-outs so they don't have to logon continuously nor do I want to risk them to lose their data if thesession has expired before they could save it.
What's the best way to do that?
I've set the time-out in my web.config to 60, but that doesn't really change a thing.
Thanks a lot.
you can increase the timeout period as much as you want, but you'll be in rist of overloading your server's memory with sessions