The trick is a Speedo and two plastic grocery bags. Seriously. The Speedo is for hygiene: Think of the many (hairy, promiscuous) surfers and divers who may have tried that wetsuit on before you. Boxers and boardshorts will just bunch up and throw off the fit.
(1) Pull the bags over your feet to slip into suits easily, and try several different brands; each has a unique shape. Also, peeling yourself out of a wetsuit is sweaty work, so don't plan to try on more than three per outing.
(2) The fit needs to be very snug, since your suit will stretch out in water. Make sure neoprene isn't piling up around your elbows or knees. The neck should be comfortable, and watch for tightness in the underarms. Swing your arms around to simulate paddling. Feel rubbing? Don't buy.
(3) Once you've made your choice, lose the Speedo.