What you see is also one of the first signs of crawling. This happens since your baby finds it easier to push away from her body with her arms rather than pulling towards her as in normal forward motion crawling.
You can also use the towel technique described
previously. Apart from this technique here are a few more things to try:
Put against a wall Position your girl in the crawling position with her feet against a wall or another stable object. I like to use a bright colored toy as some form of attraction in front. I normally position this about 1-2 feet (30-60 cm) in front on the floor. By positioning her against the wall you obviously prevent her from crawling backwards which is what you want now.
Sloping board Another wonderful technique is to use a stiff board of at least 12 inches (30 cm) wide. I like to use one that's about 3 feet long (1 meter). Now, lay the board flat on the ground and lift the one (short end) side slightly. Roughly 3 inches (7-8cm) lift should be enough.
Put your girl in the crawling position on the board facing downward. You may again use some attraction in front to get her moving and you can support and stabilize her feet at the back so she doesn't crawl backwards. What I like about this technique is the downward slope naturally encourages and assists your baby to go forward (when facing in the downward direction).
Let's assume none of the above techniques work, what do you do now?
Obstacle course Another technique I've used quite a few times is to create what I call an "obstacle course". On level floor create some kind of an obstacle course which your child can crawl through. I like to use big plastic building blocks (which I also use for other play activities), the sides of cabinets, a suitcase will also do, old cardboard boxes, etc. just to name a few. I place these reasonably close together to form a narrow passage. I make the passage sufficiently wide to allow the baby to crawl through, but narrow enough to prevent her from making an about turn.
This is usually a very fun technique and most babies like this one so much that they instinctively start to crawl... in the right direction