You know, I can't find the article with the really interesting suggestion for kneeling on one knee between each lunge. Just googling should find it. Exrx is always a good source: http://www.exrx.net/WeightExercises/GluteusMaximus/DBLunge.html

Maybe you are taking too long of a step?

I don't know; of all the exercises, I feel like lunges are my least competent, because I have to be thinking so much of my feet.