Well, contrary to what Hayes said I think it's ok to try and run outside your ends at times, just don't try to force it. If their DE gets a push absolutely do NOT try to run backwards around him, you'll lose 4 yards every time. Like he said though, you usually want to run in between the tackles the majority of the time and only run outside when it's really open. If their DE gets a push like I mentioned before, it usually leaves a huuuuuuge hole underneath him.
Don't use your burst (x button) right when you get the ball. Find your hole, then burst THROUGH it. If you burst immediately you won't have time to find the hole.
Learn the spin button, that thing can get you some huuuuuge gains. Just make sure you know how well it works with your RB's agility.
Lastly, perhaps the most hidden of all tips, make the 'light juke' your friend. If you notice, usually when you press L1 or R1 the RB makes a "hop" to the side. However, if you lightly tap L1 or R1, the RB makes more of a stutterstep to the side and then gets a HUGE acceleration burst. It doesn't look like much when you do it, but for some reason it completely destroys the defensive AI in the game, and has the added bonus that the acceleration burst can often break you out into the open field.