The most important things for properly maintaining a RO or RO/DI system are regular filter changes at 6 month intervals, disinfecting the system at that time and making sure the waste ratio is at 4:1.
The pressure gauge and TDS meter suggestions are also good, they are the only way you have of telling prefilter and carbon block condition in the case of the pressure gauge, and membrane and resin conditions in the case of the TDS meter.

You change the prefilter and carbon block every 6 months like clockwork as they are there to protect the expensive RO membrane. Contrary to what some may think, they have very little to absolutely no effect on final TDS, they remove TSS or suspended solids, big stuff like particulates and sediments as well as chlorine which will melt a RO membrane. 6 months is a good interval if you use high quality replacement filters, if you use ebay quality filters it may be more often than that as many do not have the capacity for chlorine or sediments as good filters do. I buy all my replacement filters from Spectrapure and they are on sale here:http://www.spectrapure.com/email/cus...reciation.html

I use the 0.2 or 0.5 micron prefilter as it does the best job of protecting the billions of tiny pores in the 0.5 micron carbon block. I do not use two carbons as this is old old technology going back to the days when carbon was not as refined or developed as it is today. Some vendors still use high micron prefilters, 5 or 10 microns, which forces the first carbon to act as a secondary prefilter, what a waste since carbon is more expensive than a good prefilter and needs to have its pores open to work. If the pores are fouled or plugged with particulates and colloidal materials, many vendors don't even know about colloidals and the problems they cause, the carbon is worthless for adsorbing chlorine and volatiles.

Flush kits are a waste of money. As long as you maintain the proper 4:1 waste ratio you are doing the best you can for your membrane flushing. For a flush kit to be the least bit worthwhile you would need to operate your system manually, no autoshutoff valve or solenoids filling a top off container or pressure tank with RO only. You must open the flush valve each and every time you shut the unit off to eliminate the higher TDS water surrounding the membrane in order for it to be effective. If you miss a time or two its too late and the slids have begun to solidify of the membrane and will get worse with time. Flushing on start up has no value and if your system is automated like many of us who fill a top off reservoir or Brute can you never know hen the system is starting and stopping so cannot flush.

Use your money elsewhere like converting that extra empty canister you will have where the unneeded carbon used to be into a second DI which will give you value. Dual DI is a good investment and is relatively inexpensive if you already have the 10" canister, a few john guest fittings and a refillable cartridge and you are set.

Use the TDS meter on a regular basis and monitor tap water TDS, RO only TDS and final RO/DI TDS to check the condition of both your membrane and your DI resin. Many people only look at the RO/DI TDS and could be burning through DI resin quickly because of a borderline RO membrane. To give you an idea how important RO only TDS is, did you know for every 2% decrease in RO efficiency your DI resin life is cut in HALF? That is significant. Improving a RO membrane from 90% like the Dow Filmtec 100GPD up to 96-98% like the 75 GPD Dow would really pay for itself in DI resin savings. Or installing one of the Spectrapure Select membranes in that sale flyer which are guaranteed in writing to be better than 98% rejection would soon pay for itself at $40 when you can double your DI life over a 96 or 97% membrane. You really need to know how well your membrane is working as it affects your long term operating and overall cost of ownership. My Select membrane is operating at over 99.35% rejection rate and has been that since day one over 18 months ago. My DI life is tremendous even in Phoenix with a tap TDS of between 600 and 800.