craftgrrl | Tips on Using a Jeweler's Saw?

I'm trying to teach myself to use a jeweler's saw. First task: cutting a straight line. It's not going very well. After a few millimeters, the cut starts veering in one direction or another, and I can't figure out why. The blade is facing the right direction, and is absolutely taut. (I find it's actually harder to use the tighter it is, though.... The saw keeps catching, and I have a hard time keeping up a continuous motion.) I've traced the line on the brass sheet with an awl, and am holding the saw perpendicular to the brass.

I could be using the wrong blade. I'm using 4/0 with 24 gauge brass sheet; a number of websites say I need a 6/0, but I want to make sure it's not just my technique before ordering more blades.