Once I got my DT2 running after sitting for 27 years, I had a small leak of 2-Stroke oil from the base of the cylinder. Last winter I replaced the base and head gaskets installing the base gasket dry, nothing on it. Both mating surfaces were clean. Well this year it leaked, more than originally. Once I'd installed the gasket I torqued the head bolts to the recommended 30 foot pounds, then ran it through a couple heat cycles and retorqued the head, ran it through another couple heat cycles and checked the head bolts torque again and they were all fine.

I've purchased another set of base and head gaskets and plan to remove the cylinder again this winter. Should I apply something to the base gasket this time to keep it from leaking? Grease? High Tack Gasket Sealant? Something else?

I've been using a thin coat on both sides of the intake and base gaskets of the same special gray semi-hardening sealer used to assemble the center cases... Yamabond, Permatex MotoSeal, or Three Bond equivalent. (Pretty sure Three Bond makes Yamabond)

I pressure test after fresh assembly and without a little sealer on the base gasket and reed valve, I've almost always had a small air leak or two right after assembly... especially the reed valve intake. In my experience, air leaks never fix themselves, they get worse with time.

Ditto on what Mark said.
Are you using Yamaha OEM gaskets?
I've had trouble with aftermarket cylinder base gaskets on 2 of my RT3s installed by the previous owners. Replaced them with OEM.

Hmm...
I bought the gasket off eBay and don't believe it's Yamaha branded.

Brzn you can go to a NAPA store and buy some good gasket material and make your own and use yamabond that’s what I do 

Probably not what you really want to do--but on my YZ490 it used to suck in the base gasket along the side at the transfers.
So bought an old stock roll of gasket paper [cheap] slightly thicker than original & made a gasket a bit larger sticking out along the sides so when tightened down it left a very slight step & theoretically couldn't get sucked in.
Worked for me & wasn't that noticeable unless you got down close for a look.

Which Yamabond do I want for the cylinder base gasket to crankcase? On eBay I see there's a Yamabond 4, 5 and 6B. Then applied to both sides of the gasket?

I use the semi-hardening which I believe is #4

Permatex Motoseal is in stock at auto parts stores near me and is what I've used the last several years. 
 

This is not a sales Ploy at all, but i use this on all paper type gaskets, I have found it very good. On some of the cheaper non coated paper gaskets, i give it a good soaking as it draws in in much more.

www.pjme.co.uk/acatalog/Loctite-Gasket-A...ay-1557.html#SID=655

I am sure you may find it Local to you

Indian Head