'Is there anything I can do to be more assertive?'

What are the things that everyone in a relationship should concentrate on?
A bunch of self-help books should answer your question, but the gist of everything they say usually focuses on both partners being in love for a start, followed by a sense of mutual respect, the ability to com-municate about every-thing, the importance of honesty at all times, and a sense of commitment that keeps them together through thick or thin.

I tend to be really quiet, which becomes a problem when I'm with friends because they assume I won't have an issue problem with any of their decisions and get me to go along with anything. That doesn't bother me so much because I have been with them for a long time now. It is a problem with my boyfriend, because he has started behaving the same way. He now assumes it is perfectly okay to decide what movie we'll watch and which restaurant we will eat at, because he knows I won't say anything. Is there anything I can do to be more assertive?
This habit of sitting back and accepting anything that is thrust towards us often comes from the way our parents or siblings decide to impose themselves and their opinions upon us. I'm not saying this is why you are the way you are, but I think you have to try and figure out what makes you sit back and accept everything without wanting to put across your own opinion. As for your boyfriend, the only thing that can stop him from completely taking over all decisions is your ability to tentatively make suggestions instead of accepting everything he does. If you don't like something, say it. Put yourself first for a change; you owe it to yourself.