Wake to sleep is talked about in the baby book. A good place to look might be on the sleep board--either general sleep issues or night waking. Basically you go in about .5 to 1 hour before the usual wake time, do something to get your lo to partially wake but not all the way, and resettle. The theory behind it is that if you interrupt the sleep cycle they will have to restart it which either breaks the cycle of waking up and/or extends sleep times. It certainly is not for the timid so I would recommend trying to find out as much as possible about how to do it before giving it a go. I've always been too chicken to try it myself!
Otherwise you could try going to one nap by pushing the am back 15 minutes every 3 days, keeping the pm nap at the same time or close to it, ut makeing sure you wake lo from pm nap at usual time so that the bedtime doesn't get shifted later.