As promised! I'm never going to win a prettiest sump award but...
Main things that have worked for me for 5 years with no water changes between my former tank and current one are:
1) CaRX - must-have for rock solid alk and Ca levels. Mine also keeps my Mg in check pretty well and dare I say, some other trace minerals from the melting of the media. Something you don't get with 2-part in my opinion.
2) Aggressive nutrient removal via monthly GFO (phosphate) and carbon (organics) changes, running a good amount of biopellets (nitrates), and an over-sized skimmer at roughly 2x the stated capacity. My Reef Octopus is rated for 500 gallons... my total water volume is about 250 gallons.
3) Make sure that RO/DI is giving you pure water! I change all my filters and DI resin every 6 months like clockwork. I've had outbreaks of cyano, diatoms, dinos, and other microalgaes just letting the filters sit too long. When RO/DI water is the main addition to your tank daily for years, let's make sure its pure.
4) Test, test, test! I'll admit it, I'm a testing fool! Blame the former scientist in me. In the initial, as you prolong your time between water changes, monitoring nutrient levels is vital. As you see peaks and spikes associated with nutrient levels disappear, you can slowly omit water changes. It's easy to dial in your CaRX immediately, set it and your alk/Ca stay pretty locked in. But running biopellets, heavy skimming, and GFO usage gives you all these peaks and valleys in nutrient levels. As things start balancing out and your bioload stabilizes as well, you'll see little change in nutrient levels and at that point is when I cut the cord and stop doing water changes... granted I keep monitoring levels to make sure it stays stable.
4) Feed like there is no tomorrow! My tank gets fed 5 times a day and a heavy feeding at night. We're talking animal planet special on night dives over coral reefs feeding... the water is drowning in mysis, worms, pellets, flakes, nori... Etc. Just make sure you ramp up the feedings slowly. Don't start dumping 5 cubes into your system and think that's not going to affect your nutrient levels at all. I'm a firm believer in heavy import, heavy export for my reefs.
Hope that helps!