Getting the price right and having it matched is the secret, but not always so easy to do. As Chrome said, laying at over the odds will always lose eventually.
I remember a graph I had up on goggle once about English Betfair that showed layers and backers were about even in end results of profit or loss over a 12mth period.

Pengo,
Because a race has 12 horses it does not mean they all have an equal chance of winning or losing. That's just plain nonsense and setting your price to that theory would be suicide.