I've done this in three ways. Two were at General Admission (i.e. standing room only) concerts, the other was at an assigned seating affair. No matter where my starting position or ticket, at the 5 concerts that I've been to that qualify, I have *always* gotten to either the first/second row of General Admission seating, or less than 5 rows behind the pit of a ticketed concert.

1. Jockeying For Position Early
This is pretty simple. Before the concert starts, especially while the gates have just been open for a few minutes, people are milling around in loose groups. I emphasize loose, because it means there's lots of wiggle room in which to squeeze past people for a better position. However, if you do this, you're in it for the long haul, as you will most likely not be able to go for food, the bathroom, without relinquishing your position. This method is for the "hardcore" fans, and those who have gained their spot in the front like this hate those who get their spots at the front through pushing and suchlike. However, I say that if you must fight for the front row, you're more of a fan than the wimps that didn't want to break a nail.

2. Following The Guys Bigger Than You (aka Trailing)
Again, another obvious title. This can happen before or during the music, which makes it all the much more challenging. The concept here is that if you notice a group of guys trying to squeeze their way past you, you join them, following them as far as they'll go. As they push their way through the crowd, they will undoubtedly leave a small path through which you can follow to fame, glory, and a better view. The beauty of this is that if your "group" stops in the middle of another group, they will tell you to get past them so that they aren't so crushed. Take advantage of this!

And now, my favorite...

3. Rowhopping/Evading Security.
I used this at the 2002 Y100 FEZtival, and I managed to get from Row P on the very very left end to four rows from behind the pit, with a perfect near-center view for the best concert I've ever seen in my life. I shit you not, and here's how:

First off, some background. This event took place at the Tweeter Center in Camden, NJ. The rowhopping took place in the Main Stage, which was called the Pavilion, IIRC. The concert was a true festival, and the Main Stage had 9 acts. Early on in the day, for Sugarcult, Unwritten Law, and Pete Yorn, it was general seating, which meant that you could sit anywhere as long as whoever held the ticket wasn't sitting there or didn't show up. After Pete Yorn's performance, they emptied the Pavilion, and it now became ticketed (i.e. you sat where your ticket said you were allowed to sit). So, I get to my seat, which was Section 104, Row P, Seat 32. Literally, the left end of the row, and a horrible view. As I sat down with my bag of swag, I noticed a few guys around my age behind me, quickly advancing down the rows. As they passed my row, I was determined to follow them. So, I got up, and started hopping the seats along with them. We made it to the front of the aisle between 104 and 103, where we stayed until some bitch sitting in the front row of Section 103 complained to security. Security was already clearing the aisles on a regular basis, and when they came, I did the smartest thing I've ever done at a concert:

I hid in a seat

Yes, it's that simple. I ducked into row 4 (can't remember the letter) of Section 104, with a perfect view of the stage. Security didn't notice, so I stayed there. I stayed there through Jimmy Eat World, Our Lady Peace, and Papa Roach. the section did fill up, but the owner of my seat didn't come. However, after Papa Roach, my throat was raw and I required a bottle of overpriced water. So, I entrusted a guy who looked trustworthy with my swag bag (which, for the record, contained posters, cd samplers, and my sweatshirt which I had taken off earlier in the day), and got a bottle of water. When, I came back, security was checking tickets. This brings us to the final way to reach the front at a concert:

4. Bullshit security
Yes, It's Just That Simple. When she asked me where I sat, I was truthful. What I did say, however, was that while seating was still general, I had left my tee-shirt in an unknown seat in the fourth row and forgotten it, and can I please go and look? Guess what? Worked like a fucking charm. I laughed all the way back to my seat and the guy, who returned my schwag bag unharmed. I then proceeded to watch amazing sets from Incubus and System of a Down before walking to my dad's car, half-deaf, throat raw, and happy as I'd ever been.

And just so you can understand the magnitude of that feat, the seating chart of the Tweeter Center is available at http://vptc-le.clearchannel.com/seating.php