Don't try too hard. If you constantly say stuff like "Dude, I hate girly girls!" and change very quickly, you'll look like a total poser, and nobody will want to hang out with you. Change slowly, and just act how you feel. Besides, if you are comfortable being a tomboy, you won't have any problems with people who want to be girly-girls. Everyone should be who they are, and if you don't like someone who is girly, don't point them out and say you hate them because they're girly.
Your parents may get upset, particularly your Mom. If she's really girly and likes hanging out with you (like mine), she may be upset you're changing. Don't worry, just explain you're changing but you're still the same person.
Never wear anymore of those cute little dresses, it will ruin your tomboy rep.
If you've seen boy's jeans, they're a little ripped up, not hugely ripped.
This can be a hard lifestyle. You'll have to take girly girls making fun of you. Just ignore it or come up with a witty response.
Don't be flat out insolent to girly girls, it makes you a jerk.
If you've already been tomboy for a while,some of your friends might forget that you're not just one of the guys. If that's the way you want to be viewed, that's fine, but it can be embarrassing if you're in a public place and they say something. Another thing you can not just skip from tomboy to pretty. That will throw your friends off. They will be a little curious.
You can still be friends with lip gloss-loving, pink colored super make-up girly girls as long as you don't become one.