DVD's are not indestructible.  In fact, with general usuage most DVD's will develop skips and scratches.  So what do you do?  Don't despair! 

First, clean your DVD.  The best thing is to use a soft cloth and window cleaner.  This will usually take care of most surface cleaners.

Second, try a DVD cleaner product.  There are many commercial products available to clean your discs.  Keep in mind this will only take out surface scratches.  If you have small children that handle DVD's these products will probably not work.  Don't spend a lot of money on these kind of items.

Third, have your DVD professionally cleaned.  If you look on Ebay for DVD repair you will find me and others who offer this service.  You will pay between $3.00 - $5.00 per disc but that is much cheaper than a new DVD.

Fourth, check your disc for deep scratches.  If you can see light through your DVD it is unrepairable.  It means that the data has been scratched.  At this point you will need to replace your disc.

All this applies to Music CD's and XBOX cd's.

Good Luck