X=acto blades, nice new blades make for nice clean cuts, keep the new blades for the edges that may be seen, us the older blames that make a little rougher uct for the edges that you are going to glue. I just finished a foam ironman helmet http://www.therpf.com/f24/another-i...er-one-working-foam-helmet-111277/index5.html
I like to make mine out of seperate pieces, That way you can get everything symetrical. Just make card plans fo one side and flip them over to cut out the other side. If you look at my helmet once I have glued it I scored along the glue lines that needed to have grooves. It is also easier (in my opinion) to curve and form the helmet if you use seperate pieces rather then fever bigger pieces and score the pattern on.
I would normally use a hot glue gun for gluing foam, but with the helmet I wanted and needed instant bonding, because of the curves and angels I didn't want to try and hold the shape for too long while I waited for the glue to cool down and dry. So I used super glue for the helmet. Also I made most of the pieces and then taped it all together to make sure it would work before glueing, I even used cocktail sticks to help hold the pieces together while testing fitting and while making the next template for the next part. And use a hair dryer or hold the foam over the stove for a few seconds to warm it up to curve and mold pieces. Don't use a heat gun, I found that the heat gun would heat it up to much and warp it. Say you have heated a piece up and now mot of it is right but one part needs a bit more molding, a heat gun will warm it all up to much and then you are right back to square one. a hair dryer is easier to heat it up just enough. Also have a bucket of cold water there or the cold tap running, so once you get the piece heated and molded to the shape you want it, dip it in the cold water to instantly cool it, you don't want to be holding the piece for a few minutes until it cools because you may end up with finger indents in your foam piece.
Hope some of this helps. Oh and get a nice thick metal ruler, not a thing one, the knife can easily slip over the edge of the ruler.