Two passes for me.....first cut very light just to score through first layer of paper. once scored i remove straight edge or plan i am scoring along and make final clean cut all the way though making sure to keep cutting blade as close to a true right angle as possible (very easy to cut with a slight taper which will make for a poor looking finished product). Also are you sure your blade is both sharp and free of ANY adhesive? Just to give you an idea I will go through 10-12 #11 exacto blades per an average foamboard scratchbuild, the paper is very hard on the blade and will dull it quickly. I keep one "sticky" knife (for any and all cutting that may leave residue on blade such as adhesive backed velcro) and one "clean" foamboard only knife which gets its blade changed often.
small whetstone, use it constantly, just a couple swipes on each side usually does it, even with new blades. Foamcore will dull your blades just by looking at it. Practice on scraps, you'll get the feeling.
<------has a full sized grindstone.....just thought of one more thing...if by chance you are a southpaw like me you need to make sure you are buying "Lefty Approved" blades only.....exacto has a whole line of lefty blades just for guys like me