I had an MRI just last spring for a series of jaw issues I was having. Depending on the type of MRI you get it can last from 30 minutes to several hours, and you have to lay COMPLETELY still during those times (im guessing you knew that though). Mine was just for my head so it only took about 25 minutes or so. If you're getting a full-body MRI (and i'm assuming you are), you will probably be in there for a lot longer than I was. So yeah, go to the bathroom real good beforehand.
Oh, one more thing. It's VERY LOUD. Ask them for some cushions for your ears, or else bring earplugs of your own. The place I went to provided me some nice cushions for the sides of my head while I laid in the machine.
Also leave all your technological stuff outside before you enter the MRI room. Anything magnetic will literally almost fly out of your hand and onto the machine, like yoda's hiding behind a curtain in the room or something. The magnets in the machine are very powerful and they will royally screw up any kind of electronic devices you have on you.