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It's a good idea to shoot your roommate a text regarding what you plan to do decor-wise before you start buying decor. This way, you'll know how your roommate feels about dorm decor and won't have the hassle of returning unneeded items. Phrase your texts in a way that is conducive to open communication: don't use leading phrases that will make your roommate feel too awkward to speak up! This is also a great opportunity to discuss basic items that you'll be able to split the cost of: microwaves, minifridges, TVs, mirrors, and speakers are some examples of items that roommates can easily share.
When both roommates feel strongly about going a certain direction decor-wise, there's potential for conflict. Instead of trying to convince your roomate that your ideas are better, try to find a middle ground that you can both agree on. For example, if your roommate doesn't want you to use LED strip lights, ask her if she'd be okay with a neon sign, which gives off less color. Likewise, if your roommate wants to match decor and you don't, try agreeing to match a few pieces, like a pillow, lights, and a blanket, but stand your ground on your favorite pieces, like your bedding.
If you don't know your roommate well or at all, it's a good idea to hold off on potentially offensive decor, like posters and tapestries featuring suggestive art, alcohol, or profanities. Remember that you and your roommate are sharing a dorm, and you both need to feel comfortable in it: as much as you might love a sassy decor piece, you need to make sure your roommate will feel just as at-home in your shared room as you do. The odds are that your roommate will be totally fine with an edgy poster or two, but you need to make sure you've come to an agreement first.
After you and your roommate have split up your dorm room into each roommate's "side" as well as shared spaces, make sure that you stick to these physical boundaries. This means that you need to keep all your furniture, rugs, lights, etc. in your spaces. This is especially important because your roommate might feel too awkward to speak up until you've seriously violated her personal space—which will cause a much greater conflict than if you simply respect each other's boundaries. Remember that one "okay," such as agreeing that visitors can sit on both roommates' beds and chairs, isn't an all-encompassing "okay" and that a dorm needs a separation between personal and shared spaces.
This tip doesn't directly relate to decorating your dorm, but is just as (maybe more!) important. Throughout the entire school year and beyond, you need to be a clean person—meaning don't leave things on the floor, make your bed, don't eat stinky food, etc. Just because your dorm is beautifully decorated doesn't mean that you can slack off on maintenance. Making sure to sweep the floor, wipe surfaces, and stay generally clean goes a long way not only in hygiene, but in fostering a respectful relationship with your roommate.