First, the do's: Make sure your teen has a space to work in, with a minimum of distractions and with whatever supplied he need, such as a dictionary, thesaurus, pencils, paper, computer, etc. Make sure that your teen has some kind of planner or notepad to write down the homework assigned each day. If your teen's homework needs to be done on the computer, make sure that the computer is located in a commonly used room, not the bedroom, so you can check that your teen is really doing homework and not playing games or worse during that time. You may want to get a filter or blocker for your computer to keep inappropriate material off your computer and away from your family. Try to set up a schedule with your teens that ensures that they finish their homework before calling friends, watching TV, or getting on with the rest of their life.
If possible, be available to answer questions your teens may have about the instructions or to go over the work you teens have done. You can give them a hint or refer them to a source where they can find the answer (an atlas, the dictionary). If your teen has friends who will really work along with him or her and not be a distraction, you can suggest they come over and do homework together (you can provide a snack). Your teen is less likely to become frustrated in front of a friend and another advantage is that you get to see who your teenager's friends are.
Now some don'ts: Don't belittle your teens' teachers or the homework assignment; rather, try to point out its relevance to the real world. For example, homework helps to teach teens how to meet deadlines, which they may have to do when they get a job, and the math they're learning now may help them in the future when they want to figure out mortgage payments on a house. Don't do your teens' homework for them; aside from the fact that they won't learn the material this way, you're teaching them to shirk their own responsibilities.