Reset your iPhone
Sometimes a app just hangs and doesn't respond anymore. But a full reboot of the device takes some time and is maybe more than would be needed. When something like that happens I try a reset first. Thats the equivalent of killing all tasks in a taskmanager. It's super fast and solved my problems in 9 out of 10 cases.
On the old 2G and 3G this worked by simply holding down the Home button for about 5 seconds. The device jumps back to the Springboard and everyone is happy.
But when I realized that this didn't work on the 3GS anymore (because holding down the home button opens the voice rec software) I was pretty annoyed at fist. Until I found out that Apple changed this in the 3GS. On the 3GS you have to hold down that Wake/Sleep button until the "Power off" slider appears and than hold down the Home button (let go of Wake/Sleep) until you're back on the Springboard.
Apple really has a habit of keeping those little tricks a secret. I would love to see a small handbook or introductory app coming with every iDevice where you can learn all those little gestures and routines.
I found out that out through some lite version of iPhone tricks I believe.
But did you happen to read the entire user guide? You can find some neat little tricks there too
unless you don't mind paying for the iPhone tips and tricks app, then you could try that.