If your wife's weight concerns you, either for health reasons, attraction, or both, here are some steps to use to encourage her to lose weight.

[edit] Steps

Sit down with your wife and explain that you have been thinking about your health. She will be much more open to the idea of exercising if you do it together. Tell her that you feel you could be in better shape and that you'll need her support. Try to get her to go along with the same plan.
Find an activity or exercise program that both of you can enjoy together. Walking is always good. Some other fun ideas are fitness kickboxing, pilates, dancing, tennis, and skating.
Set goals...pounds, inches, or just to be healthier. You pick yours, and let her choose hers. For incentive, set up some rewards for when you each meet your goals.
Give yourselves awhile to adjust to your new workouts. Change takes time. Compliment her on how hard she works and how great she looks.
Keep it up. Don't stop encouraging her and supporting her.
Try to include more fruits and vegetables into your diets.
Spend weekends doing fun physical activities. (Go to the park and play frisbee with your dog, go to a lighthouse, visit a museum, etc).

[edit] Tips

Cut down on the sweets, too. If you make changes with her it will be easier for her to succeed.
Encourage her to hang out with her friends. She'll need some time to chill out and not worry about exercise.
If you think she'll go for it try saying something like "Honey, I've seen the way guys look at you when we're out and I'm worried. I think that you should take up a self-defense course so that no guy can get his hands on you".(Don't quote me; change the phrase to your personality).

Do not be too pushy. Odds are she feels worse than you about her body shape.
Don't forget to tell her you love her. She needs all the support she can get.
Do not leave her to work it alone. Get involved in fun activities together (biking, swimming, walking, etc.) Set goals for yourself too.
Don't make it about the pounds. Try to exercise to be healthy and fit.