Ahh, it just takes some practice... You can modulate the accelerator using just the X button, or, use the right stick for your accel/brake.
I just use the button and have gotten half-assed decent at it. Just pick a spot on the tach, and hold it there. If you've still got wheel spin, decrease the pressure. If you don't have any, try and increase the pressure a wee bit.
It seems to me (from "real life" experience), you want to just light 'em up for a sec, then wait for the tires to hook up and you should get a decent launch.