X-acto blade works great for me. You just have to watch out that you don't let the balde follow the grain or veer off your intended cut line due to variations in wood density. Rather than trying to make the cut in one pass witht he blade, I score it lightly a couple times to really establish the cut line before cutting all the way through.

Also, blades get dull. I prefer to buy them in large quatities so I have plenty on hand to change them out often.

A good, solid, heavy straight edge helps out too.