Dress for the night or two of waiting for rescue after treating injuries. Consider how you'd dress if you knew you'd be pinned in your seat that long.

While a parachute sounds like a good idea, it is best applied to inflight fire or structural failure situations which aren't very typical. I suppose you could make the case for them with fuel exhaustion too but.... it would be hard to jettison a pretty good glider at high enough altitude for the 'chute to be useful.

Nomex flight suit including gloves and helmet would be of great comfort sometimes but ultimately folks would make fun of you when you climbed into your Cessna 172 dressed that way. Even CAP isn't wearing nomex or helmets for their missions.

Unfortunately, most of the light aircraft fleet isn't very crashworthy from the standpoint of restraints and crushable structure. Primary reason is that most lightplanes are really OLD. Shoulder harnesses, for example, aren't universal. Then again, it's hard to make something light enough to fly if it needs to be crashworthy for 150mph impacts.

You can look over the many crashes over the years and get a sense of errors that lead to disaster here:
http://www.ntsb.gov/ntsb/month.asp

An awful lot of the problems amount to stupid pilot tricks or known traps that keep catching pilots since the 1920's.

Just watched a YouTube last night of a Bonanza that flew into clouds in a pass and the videographer riding in the backseat of the plane caught the flash of greenery thru the windshield as the plane's right wing touched bushes on a hillside parallel to their flight path as they emerged from a cloud. They survived and the plane flew to the airport where the wing damage was shown. Classic stupid pilot trick which isn't always survived......