Follow him or her
Constantly. Nothing makes a relationship stronger than spending 24 hours a day, seven days a week with them. They will probably really appreciate your efforts, but will not want you to know that. Therefore, they will draw back from you, but this only means you should shorten the leash and get even closer.

Share a toothbrush
It really brings you together (especially when they don’t know).

Do things to slowly drive them mad
This could be a range of things and/or activities, so really make this your own. One possibility is slowly switching their coffee to decaf, and then after several weeks of decaffeinated coffee, put espresso grounds in the mix. Not only will they love you more, but they will have so much more energy to do so. Something else you could do is make the most annoying noises possible. Chew loudly, click your tongue after every word or whistle in high-pitched tones.

Act like a couple in public
Maybe even merge your names together to make one of those cute couple names.

Be super awkward
People love that, right? I mean, that’s how I managed to snag my boyfriend, so it must work. When your waiter tells you to enjoy your food, tell them “you too!” If someone is walking towards you on the sidewalk, get in front of them and do the awkward tango of trying to figure out who should go where to avoid collision. Do anything that would make anyone uncomfortable. Like the first object of the list, this will only make your roomie-relationship stronger.