There is an annoying problem with the current Android version where if apps are installed to an external card the app icons reshuffle and move every time you restart your galaxy s2 or use a task killer. You tend to end up with a lot more screens than you had before and because your icons get moved, it then stops you finding your apps at a glance.
I have found a way to resolve this but only if your apps are stored on internal storage. You will need to navigate to Applications, Settings, Applications, Manage Applications and then click All. Scroll to TwLauncher and click on it. Clear data and cache (this resets your home screen to original settings but all your apps and app screens will still be there). Now you will need to restart your phone and then set your app icons as you would like. They will now stay in place.