Many racers take some form of life jacket now. Racers have come up with an amazing variety of methods to keep themselves afloat if they dump their boat and ingenuity is rewarded in the Classic with weight savings, versatility and comfort.
The most obvious first choice is the basic kayaking vest. Vests are comfortable and will keep you warm when you're floating. They can double as a sleeping pad and if you have a good sized pack you can also use them as a back support. Vests usually have good flotation capability but they also weigh quite a bit for a fanatically light-weight race and they are very awkward to carry in a small pack. Still, if you're looking for maximum flotation safety, this is the way to go. You won't be sorry you brought a vest when you're floating big water in a small raft. You will be sorry you brought a vest when you're thrashing through slide alder and thick undergrowth and the vest you tied to the outside of your pack keeps getting snagged on branches.
There are a couple different types of inflatable life jackets. Inflatable PFD's are a little more svelte than full sized vests, so they will save you space but they are just as heavy because of the CO2 cartridge that fills them with air. You can alter most inflatables by cutting off the cartridge and extra fabric to make them a little lighter. Just make sure there's a way to blow it up manually before you go crazy with your scissors.
Even better is an inflatable belt. Belts are about half the price of inflatable jackets and weigh a lot less. Buy one that allows manual inflation and cut off the CO2 cartridge and you'll have a skookum flotation device that weighs about 8oz and takes up hardly any space in your pack.
A few racers have used box wine bladders as a creative flotation solution. Buy a box of wine and empty it. Wait until the hangover subsides and you can use the bag as a flotation device. It's strong, it's light and it packs down to almost nothing. Tuck it inside your clothes or maybe tie a few of them securely together. Use your imagination. Preferably though, put it against your chest or stomach and not your back, otherwise you might end up floating face down in the water when you're knocked unconscious by that nasty overhanging sweeper.
This list isn't exhaustive but it'll get you thinking in the right direction. There are other better methods and designs that adventure racing veterans regularly use. Just think light, strong and versatile.