My little sister got married a week ago. As someone who has been successfully married for 3.5 years, I'd like to address some advice to her.
When I say I'm successfully married, I mean I am still happy to be married to my husband and I believe he's still happy to be married to me. I do not mean that we have not had moments of meanness, disappointment, frustration, or bitterness. Dear sister, your marriage, like all human relationships, will never be perfect. But never fear, committing your life to someone you love will also bring your life more joy and fulfillment than anything else I'm aware of.
The culture of Mormonism that we grew up with has a very idealized version of marriage. While there is something to be said for shooting for an ideal, I know that in my experience and the experience of many others I know, this has led to a somewhat naive perception of what it is to be married. The hardest thing about my first couple years of marriage was addressing misconceptions I had about what it would be like. Keeping your eyes open early on will help you develop healthy ways of dealing with the inevitable struggles that come with matrimony.
Marriage is hard. Ask any married person to be completely honest with his or herself and they will all admit that they occasionally want out. This is 100% normal. You will not have the freedom you did when single or dating. All decisions you make will now effect a partner and not just you. All decisions he makes will effect you and not just him. You will not always agree on what decisions are best. You will fight. You will have to make sacrifices.
Make decisions now, early in marriage, about what you will sacrifice and what you won't for your husband and hold to them. Do not allow yourself to be lost to the unit. Having a partner in life will make everything more meaningful; the unit is a wonderful thing. But it is not the only thing. You are more than your husband's wife. Have goals and activities that are different from his. Encourage him to pursue goals and activities that are different from yours. Only you can allow yourself to feel tied down by your marriage. Having separate pursuits will also make you more interesting to each other, which helps keep the romance alive.
Being married does not mean that you or your spouse will magically stop being attracted to other people. Objectively, it makes total sense that this would be the case, but for so many people in their bubbles of we're-so-happy-we're-getting-married-and-the-rest-of-our-lives-will-be-perfect, this may hit you by surprise. It is better to be prepared for these feelings than caught-off-guard. Talk to your husband about what you're comfortable with and what you're not as far as interactions with other people of the opposite sex. This will help reduce risk of jealousy and/or infidelity.
Plan for time together. Just living together isn't going to cut it, as life has a way of keeping you busy even as you share an apartment and share a bed. Regular date nights are a terrific way to enjoy each other's company. You'd also be surprised how much things as simple as "we always watch this TV show together on Thursday nights" foster a feeling of togetherness.
Regularly address your relationship and what's going well and what can be improved. It's cliche, but it's because it's true: communication is the key to a successful relationship. If you're unhappy about anything in your relationship (this will happen more than you think...not to keep ringing the marriage-is-hard bell), learn to bring it up in a positive way and not as a personal attack on your husband. "We can get the bathroom cleaned faster if we do it together" will go much farther than "You never help clean the bathroom." When your husband brings up things he is unhappy with, try not to take it personally, but see it as an opportunity for relationship improvement (I'm terrible at this one, but it's still good advice).
Despite what much in American and particularly Mormon culture may tell you, marriage does not equal happily ever after. Rather than be disappointed, embrace this! You still have the majority of your life ahead of you. Do something meaningful with it, keeping your husband by your side.
Good luck. You've got this.