Here's an article you might find helpful: www.sparkpeople.com/resource/nutrition_art
Do you change your exercise routine frequently? After about 4-6 weeks of doing the same thing all the time, your body starts to get used to it and the activity isn't as much of a challenge. So it's important to vary your routine, and I'd also recommend adding strength training 2-3 times per week if you're not already.
Hope that helps,