The first thumb rule you want to learn about when deciding on a roommate is, you should be comfortable with that person regardless of which sex he belongs to. If you do end up with someone of the opposite sex, do not reject him or her outright for his or her sex. If they are willing to share it with you and you are comfortable with them then go ahead. But be sure to learn and share a few ground rules before you set out on that endeavor or it won’t be a very easy life especially if you have always lived with people of same sex. Surprisingly having a sibling of the opposite sex helps in ways of drawing lines but then again never try to do the things you have done on your sibling on your roommate.
Both of you need to be clear on your relationship status with each other. It helps clear a lot of ground. If you feel your partner will not be comfortable with you living with a person of different sex then it is probably best to reconsider. Also any feelings of attraction felt towards your probable roommate is best not be tested out by making them your roommate.
Hygiene becomes crucial when people of two opposite gender live together. While one may be a stickler for cleanliness there can always be common grounds you can meet at. The best policy to have when you share a washroom is to not leave behind any trace of having every used it. This includes not crowding the cabinet or sink with innumerable bottles of bath care products and also removing traces of any form of shaving having ever having taken place in the closed space.
What each other can wear in the common space of the apartment is also another crucial yet dicey area that needs to be discussed. If you are uncomfortable with a man without shirt sitting in the dining room then let it be known. Conversely he too has every right to say he is uncomfortable with you working in the kitchen in your night wear. For best results keep a shirt or shrug handy in your room before you leave it.
Men and women have different emotional quotients. As roommates you will have to cope up with that from time to time so allow each other enough space to know and interact with each other easily.