First rule, of course, is to avoid checking valuables (particularly those which are irreplaceable), if at all possible. What was the item?

TSA actually has a claim form, although no experience with it.
http://www.tsa.gov/traveler-information/claim-forms

Depending on the item, you could file a claim with your airline.
Another option would be to file a claim with your homeowners insurance, if it's covered.

Basically, tough to prove that TSA was the reason your item was damaged. Did you have a little TSA paper in your bag? Even then, it's possible it was repacked intact, but broke due to less-than-gentle baggage handling, or just because it was repacked improperly, or packed improperly in the first place.