I managed to get my hands on a Tamiya branded handle that uses Olfa blades. It cost me a lot more than good old Xacto, and the guy on Amazon I bought it from had to ship it from Japan, but I'm quite happy with it.
A friend of mine works at a plastics plant and they used to bet blades that fit in an Xacto handle that were much sharper and longer lasting than the Xacto ones. Not sure what the name was though. The blades came in a box that was several inches on a side so likely a life time supply for most casual users.
I'll have to ask they might even be the blades he was using. I think he still has some although they've changed techniques and he no longer has to trim parts.
Plain old Stanley and some decent quality small edge cutters. Not that I've made a model in years, but I have a few staring at me from the cupboard making me go 'hmmmmm' of late.
I know I am late to this thread, but I use disposable surgical scaples. They are cheap, very sharp and fit nice in my hand. You can get them at most medical supply stores. Some hobby shops even carry them now.