"Always set your speed before the corner" is an oversimplification. Sometimes the fastest line calls for trail braking through the corner, this might be the case for fast s-turns on grass or tacky dirt. Other times, you'll want to get on the gas as you come through the corner, this is more common in tight, slow switchbacks when traction is abundant. Other times, if the ground is very loose, you need to tip-toe carefully through. In any case, the only way to learn is to practice, push your limits and don't be afraid to fall down. Better to fall down in practice than in a race! Keep in mind that if your tires are not sliding around underneath you on certain corners every lap, you are not going fast enough.

More important than anything, though, is taking the right line. In cross, this is almost always a wide setup with a late turn-in. This allows to eliminate as much of turn as possible by taking a straighter line. It lets you come in faster, because you aren't going to exit the corner heading toward the tape, and it lets you get on the power again much sooner. Practice setting up as wide as the course allows, turning in late and ripping it! Remember, if you aren't sliding around a bit and you aren't occasionally falling down in practice (crashing once per race is actually kind of a bad idea, though I've done it!), you aren't going fast enough!