Round Blades or Straight?
A straight blade is best for plunge cuts, where you stab the blade into the surface. For long, linear cuts, use a round blade. You'll get cleaner cuts with the round blade if you make a shallow scoring cut first, then gradually deepen it.
In any case, keep in mind that heat kills blades. Occasionally swing the blade back and forth out of the kerf to clean out dust. And don't press so hard.