Rot forces a "con check" (basically a test of your constitution) every tick. If you fail it, your con goes down by one and you take some damage. The lower your con, the more damage you take. If your con drops to 3, you die.

However, if you have high con, you don't tend to lose it on the tick. What seems to be the critical point to me (that I never knew until someone pointed it out here), was that there is benefit to being above max con. Not only because it offsets the con you lose, but also because it appears to help you pass the check on con that happens every tick, so you don't even lose con very often.

Fortitude of flesh is a cheap edge but doesn't seem to help all that much. You still need +con gear if your con is low (and I include elf/drow at max con in the "low" category).

Some useful items are:
jerkin from arial graverobber under arkham (+3 con)
iron bracers from frigid wastes (+2 each)
amulets from goblin village (+1 each)
couple of rings (e.g. pitch black, from Udgaard, +1 each)
juggling balls from aldevari (+1 or 2, I forget -- basically hold them when you are rotted),

So that's +12 there. You need to get them on asap after being rotted, if you are low on con, and don't stick around to see if the shaman lands a -con damnation on you unless you really have a lot of extra +con. I think Nererial had about +20 or so. Maybe more.

The notable thing about all of the above is that they are not limited items adding 3 con or so. They are small items that can be packed in a girdle.