This is something that I had noticed when I was changing the fluid over the summer. I pushed through just over a litre of fluid before everything ran "clean" out of the nipples.
I guess you could do a change on one litre or two litres would ensure you had fresh fluid everywhere.
The Gunsons pressurised system is brilliant. Run off half a litre to each wheel and you are guaranteed fresh fluid everywhere. Takes about 20 mins from setting up to packing away. Read the instructions carefully and have a watering can or hosepipe handy for any spills.
We found that about 25psi in the tyre is better than the recommended 20 but you need the bigger seal on the master cylinder unit. Also you need to have someway of removing the extra fluid to drop the reservoir level down to the recommended mark. Easiest way is a syringe from your local chemist for a few pence. This will quickly extract the extra fluid without mess.
If you get, or have used, the Gunsons kit this will all make sense.