I just moved (this past Monday) into a house (37yrs. old) that is the same as you. No vapor barrier or insulation. I already notice the cold feet during these last couple days of cool weather in Maryland.
Anyway, what you (and I) need to do is put down a vapor barrier on the dirt...the floor of the crawl space. I want to do that first so I don't get so dirty.
Then, lay on your back and put insulation between the joists. I couldn't imagine ripping up the subfloor in my new house to do that! But maybe it would be easier in your case...though I doubt it.
One thing that maybe someone else can help us both with. When buying insulation, do you get insulation with paper vapor barrier attached and put this against the floor...or just buy insulation without any barrier?