If you don't have a machine and you really want to provide max protection to the transmission without taking it in and paying the ludicrous service fees (the local Ford and Toyota dealers now want to charge me $250-300 for this service!!!!!), and you don't have a trans pan plug (curse you, Ford!), then you can probably do what I do:
1) Go out to Harbor Freight and buy a fluid transfer pump. I got mine there for $2.99. Or you could get a Mityvac fluid extractor - they're VERY nice but $60-100.
2) Every year, suck as much fluid up the transmission dipstick as you can using the fluid pump. I can generally get 7 quarts out of the pan this way out of the 13.5 that the whole system holds. Then just refill the transmission through the dipstick. Cost: ~$35 for two gallons, leaving a quart to spare.
3) Every other year, do the above service, but before refilling the transmission, drop the pan and clean it and replace the filter. Sucking the fluid out first makes the pan drop a MUCH cleaner job (my car's normal fluid level is above the level of the gasket, so if you don't suck the fluid up first, it spills everywhere when you crack the gasket. Cost for this service: $35 for the fluid and $32 for a filter from the dealer (I prefer dealer filters here as it is cheap insurance over the $18 aftermarkets, and my car has multiple filters depending on VIN #, so aftermarkets aren't always perfect matches).
So that's $51 per year for servicing.
After 10 years or so of doing this, you're pretty much at a steady state condition where the fluid (by volume) averages 11 months old after a change, 23 months old right before a change, and 1.429 years old over the course of the year.
After 15 years, you still have some original fluid in your transmission. But it only makes up 0.36% of the total volume of transmission fluid. Big deal.
Do that and accelerate modestly and your transmission should last you a LONG time. If you want to save even more, I'd say you could change the transmission filter every 3 years instead of 2 without any real increased risk.