to hold the mount slightly downward as you're changing. also, if your other pieces of equipment are dusty (like your lens) it can easily transfer to your image sensor. so, also keep everything as dirt & dust free as possible.
you can purchase compressed air cans which give a nice swift blow of air onto whatever you're pointing it at. there are also bulb blowers which are easier to transport in a camera bag, are inexpensive, and are an easy way to keep those nasty little particles away.
another "killer" is fungus. fungi like to creep into lenses when you least expect it. on normal fungus levels, it is very hard to 