Several things I do:
1) I've found the absolute cheapest utility blades don't hold a good edge and dull like crazy. I bought a pack of 50 from Harbor Freight and went through 2 blades cutting out a 2 sheet plane. You don't need to buy the most expensive ones, but paying a little more for better quality or heavy duty blades makes a world of difference.
2) A Cutting mat will help reduce the dulling. Cutting on a hard workbench, metal table, even a plastic folding table, will dull the blades and prevent it from cutting cleanly.
3) Having a handled blade like others have suggested, will help you make cleaner cuts. I know you mentioned that you have one already, but it bears repeating that.
If you have one that is comfortable in your hands, you'll be able to make better cuts; some cheap utility knives don't feel comfortable or have a very thin body, and so I feel like I don't have good control - that's more of a personal preference, though, but good control will help you make smoother and better cuts.
4) Not sure if you're using them, but invest in an Exacto/craft/hobby knife with #11 blades. You can find these at just about any craft or hobby shop, or online; I would highly recommend buying one that you can replace the blades on, as you will need to change them out on a regular basis. I use these for curved cuts or small cuts - say, for example, small notches on a Mighty Mini for the control horn, or a narrow notch cut where the tip of a utility knife would end up being too wide by the time you cut through the paper. With this type of knife, you can hold it more like a pencil and get a better detail cut.
5) Be careful when you're cutting to make sure you're cutting with the blade going perpendicular to the foam board - that is, you don't want the knife to be at a 45 degree angle left or right when you're cutting, or else you'll have misshapen parts. Using a ruler will help on this, especially for the straight line cuts.
6) I didn't see anyone mention this, but if you can, get a cork backed ruler for your straight cuts. The reason for this is that the cork backed rulers don't slide like a straight metal ruler or a plastic ruler. If you're trying to do a long cut (say, for example, a long fuselage cut, or a wing cut) it helps to keep your line straight without accidentally sliding and cutting into the material you DON'T want to cut.
These are all things I do and have come up with some great scratch builds; YMMV (your mileage may vary).