I bought a special funnel. It has 2 main parts. A peice that screws onto the radiator like the normal cap, and then a funnel with stopper that sits on top of that. Easy to use, put on the cap, put on the funnel, fill'er up until the funnell is about 1/2full and start the car. As it runs and bubbles work their way out you can watch them in the funnel. When you are done you have 2 options.
option 1. wait until the motor is cooled, shove the stopper in the funnel, then pull the funnel off, unscrew the rad adapter, put the real one back on and yer done.
option 2. rev the motor a little when you think it is done, usually this pulls in a little fluid, get a friend to take the funnel of hot fluid away ASAP AND unscrew the adapter and shove your real cap on. (not recommended)
the whole funnel setup cost me 20$ (I think) from Carquest a couple years ago. I learned to love them after working in an shop for a few years.
-ron