If barking is a potential source of trouble for you -- for example, if you live in apartment or condo with noise-sensitive neighbors who are home all day -- the first step is to ask for a foster dog who is likely to be on the quiet side. Beagles, hounds, terriers and some toy breeds tend to be quite vocal. Senior dogs tend to be quieter. Younger dogs are a mixed bag: many adolescent dogs are relatively quiet, but puppies can be very loud.
The next step is management. Use curtains or blinds to cover windows that look out onto busy streets or squirrel-filled yards where your foster dog might be tempted to camp out and bark in excitement or frustration at all the goings-on. Don't leave your dog outside in the yard all day; lonely and frustrated, he might bark nonstop to relieve his boredom.
Many dogs just sleep all day while their people are gone, but others bark because they have nothing else to do. If your foster pup is in the latter group, give him plenty of exercise when you're around and leave him with long-lasting toys like frozen stuffed Kongs to entertain him for a while after you leave. Do your best to get your foster pup tired out before you leave for work, and/or arrange for a dogwalker to come and take him out for a good long trek in the middle of the day. Doggy day care may also be an option if your foster pup is dog-social and up to date on vaccinations and preventatives.
If your foster pup barks at you for attention or food, ignore his demands and reward him only when he sits quietly (remember: nothing in life is free!). If your foster pup gets over-excited while playing and erupts into spasms of barking, give him a minute or two in time-out to teach him that excessive noise makes the fun stop (but also be aware that this type of barking is difficult to quench, especially in the limited time you'll have as a foster parent).
Training a positive interrupt ("Quiet!" or "Thank you" used to cue the dog to stop alarm barking) is very similar to the "Attention, Please!" exercise, and can be useful for dogs who like to sound the alarm a little too enthusiastically. Give the cue -- I use "Quiet!" -- and click/treat your foster dog for turning and looking at you. Do this until he seems to get the idea, then practice by interrupting him in not-very-interesting activities (chewing on an old familiar bone, wandering around idly, etc.). Gradually increase distractions and move the exercise outdoors. When that seems to be working, try it when he breaks into barking. Only click/reward when he is looking at you and being quiet. The clicker can be a powerful tool in encouraging calm.