Know how much oil should be in the transmission and plan what you're going to drain it into. My transmission holds over 5 gal, so I can't drain it into an empty pail without putting the plug back in (real messy). Fortunately that never happened, because there isn't clearance for a 5-gal pail under the tractor.
I used a 12-liter drain with a pour spout on the side. When the drain started getting full, I tipped it so it poured into a smaller pan, which I then emptied into a pail. I screwed the fill cap back in to slow down the flow while I emptied the small pan. I know, just buying a bigger oil drain would have been easier, but the local store didn't have one.
As Wen said, a hand pump to transfer the new oil is a real good idea. Trying to balance a full oil pail on your knee and pouring through a funnel won't be a good time. On my front axle, you'd have to have a gooseneck funnel. I bought a pail pump from Northern Tool. It's a metal lid with a pump in the middle that's designed to replace the lid on a standard 5-gal oil pail.
Also plan how you're going to get rid of the oil. It's easier to deal with in small quantities, and your local gas station may take it at his (in 5 - 10 gallon lots). We had over 400-gallon of waste engine and hydraulic oil from a previous owner pumped this summer. It's not easy to get rid of much more than 10-gallon at a time.