Maybe it goes without saying but I'll say it anyway just in case... Drink lots of water and go to the bathroom as much as possible. That helps a lot, especially when you're feeling bad. Water can also help if you start having constipation.
In case of constipation, you might also try Psyllium Husks occasionally. Your body can become dependent on it, so don't take it too much.
I usually feel the worst on the 2nd day of a round. I try to schedule my rounds so that the 2nd day is on a day when I don't need to do anything important.