When I did my initial weigh in, the trainer who weighed me took one look at me and declared me an endomorph--he said that this means carbs are my personal enemy and that I should focus on strength training over cardio. I am just now getting around to doing some reading on this topic.
This site has what looks like a good analysis, and a quiz that helps you figure out your body type and how best to train for it:
http://www.bodybuilding.com/fun/becker3.htm
I took the quiz, and these were my results:
Traits
Endomorph: 38%
Mesomorph: 63%
Ectomorph: 0%
Your body is between an endomorph and a mesomorph.
Endomorph: Your biggest concern should be losing fat
and adopting a lifestyle that keeps it off. Strength training should be
done to get a better muscle-to-fat ratio, which will help to boost your
metabolism. Use moderate weights at a fast training pace, allowing for
very little rest between sets and exercises. Lower your calorie intake
and eat frequent but small meals to make sure you're still getting
adequate nutrition. Eliminate sugar, sweets, and junk food from your
diet. Get your heart pumping by engaging in some form of daily activity
such as brisk walking or biking.
Mesomorph: You have a naturally fit body but, to
maintain or improve it, you should adopt an exercise and diet regimen
that compliments your build. You can strength train more often and for
longer periods of time, but be careful not to overdo it. Train with
moderate to heavy weighs and keep a moderate pace, making sure not to
rest too long between sets. You'll find you gain muscle quite easily.
Don't fear; you won't get "bulky." When you're happy with your muscle
size, simply train to maintain. Stick to a good, healthy diet to stay
lean and muscular, and watch for any slow-creeping fat gains. Engage in
and enjoy aerobic activities, but be careful not to overdo it.