Coming from the Palm Pre (original Sprint flavor) I have two learned habits I've been trying to break with the iPhone 4S:
First, the Pre was notorious for having memory leaks, so you had to reboot every one to three days to keep it running. If you didn't, you would try to start an app and it would tell you that you had too many cards open (too many running apps) when you had none open. I've probably re-booted the iPhone more than it needs as a result.
Second, again with the Pre's memory problems, and its terrible battery life, I had gotten in the habit if throwing away all cards (closing running apps) right after using them. I never left apps open, since it would drain the battery and slow the device. I know, I know, I know the iPhone doesn't work that way, but I still find myself opening the dock and closing apps all the time. I'm trying to stop, but two years of training is hard to break...