Be open and patient. I'm sure others can add more to that, but that's essentially what I've used when living with past girlfriends. The other party is going to bring over their personality, traits, and behaviors. There's a big possibility that you may never have seen these before, or you may have, but it didn't affect you because you weren't living together. Something to consider is possibly bracing yourself for the changes to come. Here are some that have come up in my past, and are good things to think of before moving in with each other or even discussing them.
1. The bathroom. There are several things that go on in here that may really turn the tide if you're not prepared for them such as: The toilet seat being left up or down, hair in the bath tub/shower/sink, cleanliness in the entire bathroom such as clothes and towels lying around.
2. The kitchen. Do the dishes pile up after every meal? Is one person "expected to cook and clean", or does that chore get split up between whoever isn't cooking is cleaning? What sort of groceries are placed in the fridge? Again, cleanliness is also a big deal here.
3. The living area. Is there only one TV, and if so, do you both have the same tastes? Decor? Cleanliness. And, uh, passing gas. It happens all the time, but probably not around each other or in dating situations (hopefully)

What's going to happen the first time it happens because you can sure bet it's not going to be the last time.
4. The bedroom. Most people think that this area would be the easiest but it really isn't. The whole issue about cleanliness comes into play here. Does one of you leave clothes on the dresser while the other hangs up their clothes. Would it bother if one did the other while you did the opposite? Decor. Such as bedsheets, art, and flowers. Sharing a bedroom is very hard to do. Most of us have grown up having a bedroom to ourselves, and have mentally painted a picture as to what that room should look like for all eternity. The problem with living with each other in an intimate situation is that there usually is only one bedroom so it's important to talk about this.
Okay, this turned out longer than I expected, but I hope it serves its purpose as a guideline. It's definitely what has worked for me, and may vary from person to person. Good luck!