I usually use a dremel tool with the "Multipurpose Cutting Kit" #565.

I use it like a router, it can carve all kinds of cutouts, slots, you name in in foam faster than you can think about it. I use it for servo cutouts, spar slots, fuel tank compartments, and pretty much any other time I need to cut a hole in foam. I find it's a lot faster, easier to control, and doesn't create the smell of an iron. But the foam dust created can be a bit annoying, so keep the vacume handy .

For screwing down servos, I cut in a couple of wood rails, and then cut a servo-sized hole between them, and screw on the servo. I do this on my bigger planes. It might be too much weight added for a park flyer though. I've just glued the servos in to the foam as well (after cutting the holes), it really isn't a big deal. If you need the servo back out, it's usually pretty easy to cut the glue with a long knife blade. You can also wrap the servo in making tape first, then glue it in. Then, when you remove it, you hack up the tape, and the servo comes out.