Groceries

Your roommate cannot be the only one buying food for the place.  You have to chip in.  If you're eating there, then you have to spend some money.  You can't depend on your roommate to buy all the food for a month and you only shop one time in three months.  It's even worse when you bring your friends over and they eat all the food too.  Get up off your lazy butt, get a job, and go buy yourself some food because otherwise you're going to starve.  It's not your roommate's problem, so don't go whining to them.  They should hoard all the food somewhere that you can't find it so you'll have to do something.  You might think it's not fair, but trust me, it's the way it's got to be.

Cleanliness

One of the first rules for being a good roommate is that you need to help clean up.  Help do the dishes.  If you have a dishwasher then don't leave the dishes in the sink.  Take the clean ones out and put them away then start filling it back up with dirty ones.  Vacumn the floor if you get stuff all over it, or even if it's just your turn to do it.  If you make a mess somewhere then clean it up!!  Getting a text that says, "Sorry, spaghetti exploded in the microwave, but couldn't clean it up because I'm late for work," is not acceptable.  Clean up the crap! It's just common courtesy.  Don't leave crumbs and spilled stuff all over the counter.  It attracts roaches and ants and you don't want that!

Another thing: keep your own stuff cleaned up.  Now, if you have separate bedrooms then your own room can be as messy as you like.  If you share a room with someone then PUT THE MESS AWAY!!  Don't strow your mess into the living room and leave it laying all around for someone to trip over and bust their head on the wall!  That leads to emergency rooms, and that's not cool.  You're paying for my hospital bills!!