TiPB has six tips on how one can fix a slow or buggy iOS device.
It's a must read. I'll leave all of it there since they came up with the list.
I just have one more to add. Turn off any feature that you don't need. If you don't need push notification, get rid of it. That might even extend your battery life. Oh, and there's the location function if you're on an iPod touch.
I think the best coarse would be to restore the device. Apple has made it easy to do that. And fast.
More at TiPB.