1) Can you afford to replace the part if it breaks. While the extended warranty isn't usually worth the money - If you know you can't replace the item if it breaks it can be good insurance for you. If it is a policy for things like phones it may be a good deal. MAKE sure what kind of breakage is covered. Is it just defects or will it cover your, accidentally, breaking it?
2) Can you wait? Many warranties will require you to ship your unit in and, then, wait several days or maybe weeks to get it back. Make sure you know how long you will have to wait.
3) Be certain of what it covers. Most warranties don't cover things like backing up your data before replacing parts. If you are unaware they might just remove your information without discussing it with you.
4) Keep in mind the credit card you used. Many credit card companies, automatically, extend warranties. Check your card member agreement.
It always hurts when you break something if you didn't choose to get the coverage, but, unless it is being used by children, or you have particularly bad luck, over time you will save far more by paying for the repairs as they happen.