I learned never accept an IOU. Chances are, they will forget, and then when they go, that's all you will get out of them. Take a check, even if it is post-dated, you can cash that and when it bounces, you can use that in a court of law to collect.
When somebody says, "Oh while you are out, do this errand/purchase this/drop this off" -- DO NOT. Don't even discuss it, just act like you didn't hear. Because if you do it just once, from now one you will obligated to ALWAYS be the one who has to do it, and they expect you do it and will whine about it.
Have a task chart made up on paper and use the "gold star" system. Roommates always forget to do their task, and they will say, "I did it last time..." like it matters they only do one thing and leave the rest for someone else. For some reason, roommates think it's childish to have a task chart, but deny they don't do their share.
Do not leave anything of value in the common living areas. Books, CDs, DVDs, clothes all seem to disappear.