Doc, one thing that can drastically affect the life of your membranes is biological contamination.
So, if you have anything living on the inside surfaces of pipes, filter casings, RO casing, etc, your membrane will give up the ghost early.
The solution is to take out your old RO membrane (the one you need to replace anyway) and your filters. Then, feed a bottle of thin, unperfumed (value brand will do) bleach into your system. I diluted a bottle of bleach into a bucket and fed it through until the output smelt of bleach. Leave it a few hours then run fresh water through to clear the bleach (I do this for a long time, as any left will really hurt your new membrane).
All this advice is courtesy of June at Gaps. She pointed out that commercial RO users on a big scale spend a lot of time on decontamination, as it's a common cause of membrane failure.
Hope that helps.
Vin