Its all in the title... how do I remove the tank? I need to drain it aswell.
My first suggestion would be to buy a clymer manual through the link at the top of the page. Supports the site and you get pictures!
I searched for tank AND removal. found: this post on gas tank removal (http://www.gstwins.com/forum/viewtopic.php?t=6707)
I thought you had to take the rear plastic pieces off, too but I could be wrong.
you don't have to drain it to take it off. just make sure that you get the screw on the tank to point toward "off"
EDIT: there are pictures showing how to remove the tank at the beginnning of this rejetting tutorial (http://www.angelfire.com/mt2/mikesgs500/rejetting/index.html)
wow. since I obviously can't read, heres the answer to the second half of your question:
You can drain the tank by sticking pieces of fuel line on the petcock on the bottom of the tank, and open the petcock after the hoses are in an appropriate container. It might take a while, but its probably less messy than trying to get the cas out the hole on top (can't think of technical name at the moment) It might drain faster if you open the top, but I would be skeptical.
Good luck