Learning to Crawl
Crawling - assuming your baby does it - normally starts between 6 and 10 months of age. Besides giving your baby regular tummy time, preferably on the floor, there are several things you can do to make it easier for him to become mobile. Do not overdress your baby, as this would hamper the movement of his limbs and joints. When possible, leave your baby's feet, knees and elbows bare, as this will give him better grip.
To help your baby get moving in the prone position, let her push off against your hands with her feet. This should help her begin to belly crawl. If your baby looks about ready to get onto all fours, you can help by bending her legs gently and lifting up her bottom. Don't force your baby into this position though, and don't try to rush her into crawling on all fours.
You can also encourage your baby's mobility by dangling his favorite toys just out of reach, or once he has started to belly crawl, moving several feet away and calling for him to come over to you. Avoid aids like walkers, which take a lot of the effort out of getting around - to babies' detriment.
You might also consider making or buying an infant crawling track (details of which can be found in Glenn Doman's How To Teach Your Baby To Be Physically Superb). The track has a surface with good traction and can be put on an incline (by placing one end on a step). Using a downward slope makes crawling easier, while the upward slope can be used later to make crawling more challenging.