This is a common problem. Do you knit english or continental? If you knit continental, you can try "combined knitting." Amy has videos under the basic techniques tab above, or you can look here
. If english, you can try Nona's method for ribbing
. They essentially do the same thing -- you're wrapping the yarn in the opposite direction when purling. (The problem has to do with how much the yarn has to travel when purling compared to knitting and both methods involve purling in a different way to adjust for the difference.) You can also try to just purl more tightly than you knit by giving a little tug.
All that said, have you looked at pictures of ribbing in books? They are never
even, especially 1x1 ribbing. (Go ahead, take a look. I'll wait.) Only machine knitting is really even. So don't sweat it too much, because it's normal. The gauge will probably even out a bit after you wash and block it too.