On this topic, there are many good editorials on the subject online...
This one is from a woman's perspective, but it does give insightful commentaries on the issue as a whole:
http://www.familyresource.com/parenting/70/446/
I recently read somewhere that a big reason why relatively new mothers lose a lot of their sex drive is because they feel more motherly and less like a lover; less sexy, in other words. Tired, and expected to take care of the children and keep the household happy and sex becomes another chore.
I'm far too young to speak from experience here, but Debra's advice of "Seduce your wife" doesn't sound like a bad idea. Make her feel sexy, like she's still worth the time and adventure you two can have together. Find subtle ways to remind her of the fun you two used to have..
Have you discussed this with her? That would help quite a bit. To discuss it in a relatively open, supportive manner, to guage her general feelings on the subject more.