Originally Posted by springer
One technique to maintain a straight line is to "drag" the knife. Hold it about 45deg to the foam, and relatively loose in your hand with index finger right on top, then just pull it down the straight edge with minimal pressure using index fingertip, only trying to get a partial cut (basically cutting through the outer "skin"). Once you get the hang of it, the knife will self align vertical to the cut. Then without moving the straight edge, make another cut with more down pressure and do this til it is through. Usually takes three strokes for me on FFF or MPF.
This technique works best for me too and I usually build planes with 2 sheets laminated together. Don't try to make the cut in one pass, just drag your knife along the cut line several times. Results in much cleaner and accurate cuts.