Quickest way I was taught:
Take the patient's weight in lbs: (i.e. 180)
Drop the last digit: 18
Subract 2: 16
And that's the drip rate for your standard pre-mix. Can't use it on really heavy or really light people. Actually used this trick a few times on a test and got the questions right.
I have yet to use it in the field, however.