1. Get to the venue early.
2. Show up to the appropriate stage 2 or 3 acts before the act you want to be up front for.
3. Start working your way towards the front by filling the spaces that become available in front of you. For added effectiveness, start on the non-entrance side of the stage. It's not always the left side.
4. Don't be a dick.
5. Don't get too close. If you're right up on the rail you'll have the added distraction of people constantly pushing you in the back.