How you deal with it will partly depend on your relationship with the person who has bipolar. The more intimate your relationship, the more difficult it will be. You will find that the first thing you have to tell yourself when problems arise is, "they have bipolar." That should help you take a step back and view things from a better angle. Separating the disorder from the person will allow you to evaluate the behavior without getting so upset with the person.
I have bipolar and I don't SUFFER from it. At times, I quite enjoy it! You mentioned trying to take control. People with bipolar, whether they are aware of the disorder or not, often try to self-medicate or take control of what is happening to them. Although what they do often seems weird or crazy to others, it is often a logical attempt to control what seems to be messing with them. You might try seeing if you can be part of the process. Tell the person what you are noticing and ask some questions. "You've been spending a lot of time in your bedroom today. Are you feeling down? Is there something you're trying to get away from?" Acknowledging those feelings could start a great conversation.
The changing moods is as frustrating (or maybe more frustrating) to the person with bipolar as it is to those around them. I would love to be able to choose when to be manic or depressed or somewhere in between. There are things I do to try to influence my moods. I'm pretty good at making myself more depressed, if I want to be, just by playing certain music or watching certain movies. Since we can't truly control those mood swings, it is very tempting to take control wherever else we can in life.
Be as supportive as you can, but don't let your frustration make you bitter. If the person with bipolar is a boyfriend or girlfriend, put serious thought into whether this relationship will work for you. My husband saw the craziness and made the commitment to me early on and it still amazes me that he doesn't run screaming from the house, never to return.