I've always gone to a store with many different types of life vests and tried them on. Find one that fits YOU, your body type and common motions well. I also really like many of the float suits and float jackets (especially this time of year).
It never ceases to amaze me that we will spend thousands on our boats, hundreds on our fishing gear, hundreds more on our electronics and shop price on the one item that could be our last and best chance for making it out of a bad spot. Remeber, its a fine line between a great story at the tavern and the lead story on the local news.