what now is best to prevent rust returning?

[daveymc29]

I have used three gallons of vinegar to loosen up the rust in the cooling system and then used a box of soda to neutralize the acidic reaction of the vinegar and backflushed profusely with water under mild pressure. Now that I have done all of this I have refilled the system with destilled water. Fine, but after a year of driving it is beginning to look a tad red again. What should I add to the distilled water to prevent, or at least slow down the reformation of rust?