The iPod touch is a part of your every day life -- it ends up jammed into your pocket, your purse, your backpack and the like. Over time the general gunk builds up; lint or dirt from your pockets gets into the scant space around your power and home buttons, gumming up the works. In most cases simply cleaning out your buttons will unstick them and return your iPod to working order.
Cleaning the Buttons
The fastest method for cleanin the space around your button is to use a clean, dry toothbrush to loosen any dust or grime. If you're got a problem with sticky film, such as from soda or candy, using a q-tip and a little rubbing alcohol can scrub off the film and loosen up the button. Don't use any more rubbing alcohol than necessary; while it dries quickly and is safe for external use, you don't want to end up with any extra liquid seeping into the cracks.
Loosen the Buttons
Once you've cleaned out any gunk and build-up around the buttons, you still might need to loosen them up. This is especially true if any debris is underneath the button. With your iPod turned off, if possible, tap and wiggle the button to get it moving again. If it still feels stiff, double-check that your button is as clean as possible.
Protect the Buttons
Once you've gotten your buttons cleaned out and in working order again, you should consider getting a cover or film to protect your iPod buttons from further build-up and potential damage. Protective covers vary in size and style; some covers are solid snap-on protectors, some are more like silicon sleeves. When choosing one, make sure it's designed to fit your generation of iPod and protects your iPod buttons.
Things to Avoid
Avoid any hard solvents or running water over the iPod in an attempt to loosen any grit or gunk in the iPod buttons -- this will do more harm than good. Furthermore, avoid taking your iPod into places or situations to get dirty or stuck. Proceed with caution if you're going to be at a beach with a lot of sand, out near dirt or mud, or handing it off to a child -- a common culprit in sticky iPods.