While many of us enjoy the functionality of app folders in iOS 4, let's face it: they're kind of an eyesore and they're not easy to visually identify. If you like to use app folders, but wish they were easier to spot without having to read the fine print, here's a way you can have your cake and eat it too.
Instead of relying on tiny folder labels to catch your eye, you can use app icons to help you quickly determine the what's inside. Try arranging your apps in rows according to their category. Place your top three apps of a genre on the left of the row, and stash the rest of them in a folder on the right. This will help train your brain to use the apps on the left to more easily identify the contents of the folder on the right.
Also, you might find it helpful (and more aesthetically pleasing) to leave the bottom row blank. When you download a new app, it will be easy to spot on the bottom row and it'll be easier to move it to the appropriate folder. An empty bottom row also makes it less of a headache when swiping between screens when you can't keep your device completely stable, giving you empty real estate upon which you can swipe without having to worry about accidentally messing up your app arrangement.
If this tip piques your interest in app placement philosophy and you want to explore deeper down the rabbit hole, you might find this recent article by Mike Vardy a good place to start.