If you have another pot, like maybe an 8qt (or hopefully one the same size, 11 qt) then I might consider boiling with 2 pots at a time. I have been doing this because I don't have a 20 qt pot, I use my 16qt and a 12qt simultaniously so I can easily boil at least 3.5 gallons at a time.
I just split everything between the 2 pots, I dump half the extract in one, half in the other, split the hops...etc. For specialty grains I usually have 3 1/2 gallons in my 16 qt, steep all of the grains in there and finally I will dump 1/2 of that into my 2nd pot and that's when I start with all of the rest.
It sounds like a lot of work, but It's really not bad at all and since i started doing that method my beers have been tasting quite a bit better (less carmelized and better hop utilization)
The only problem is cooling down both pots when it comes to cooling the wort. I just put a pot in each sink in my kitchen, give them an icebath and top off with cool water, no biggie.
*edit: didn't notice, but yay that was my 100th post!*