So I finally came across a new gas tank for my 1985 Brat that lives with me here in Michigan. What I am wondering is... what kind of treatment should I give this tank so it will never (hypothetically) rust through again, at least in my life time? Any advice will be greatly appreciated. I want to make sure its nice and protected, even though I dont drive the car in the wintertime. 

What condition is the new tank in at the moment?

Coat the outside with Waxoyl.

Is it new from a store or new used? If it's never been installed you should not have to do a thing to it. It should be pre sealed.

If used and has rust in it. Remove every thing from it and cover the holes with tape. I used meratic no acid (check my spelling) to get all the rust off inside but have been told apple cider vinegar works as well. If going with acid do it outside with plenty of air and have baking soda ready to neutralize the acid once you pour it out. (After doing it once I recommend just buying the kit) flush the tank with acetone. Then again with a clean can of acetone. Let dry then its ready for which ever tank liner you chose to buy.