Socialize, socialize, socialize.
I live with a fearful/fear-aggressive mutt monster who, owing in large part to a severely undersocialized puppyhood, would -- despite years of intensive remedial work, numerous consultations with canine behaviorists, and daily doses of SSRI medications -- still be unable to live safely in the great majority of adoptive homes. That is how much psychological damage a lack of early socialization can do. So let me say that again, loudly: socialize your foster dog. You can't change your dog's genes, and you can only influence his life after adoption to a very limited degree, but you can still make a huge difference in his mental health and future happiness by undertaking an all-out socialization program during the time you have with him.
Intensive socialization is imperative if your foster puppy is under 4 months. This is the critical period where the dog gains the most benefit from new experiences and is most psychologically damaged by isolation. At around 4 months of age, for most dogs, the socialization window starts to close. You can and should still make efforts to socialize older dogs to novel sights and sounds, but your program probably won't work as quickly or be as effective as it would be with a younger dog. (That said, unless you specifically ask for a rehab case, any adult foster dog you receive will probably not be severely undersocialized, if only because such dogs tend to present with obvious fear or aggression issues and are seldom selected for rescue.)
Socializing most dogs is very simple, although it does take a lot of time and a ton of treats. All you have to do is expose the dog to as many new people and situations as positively and safely as possible. The goal is to teach the dog that not only are new things not scary, but they are actually pleasurable and should be regarded with happy anticipation.
In most cases, this is most easily accomplished with loads of tasty treats, but you can use whatever rewards your foster pup finds fulfilling: off-leash play with other dogs, walks in places he likes (forest hikes are a big favorite with my crowd), ball tosses, Frisbee throws, whatever your imagination can devise.
Link those rewards to the new stimulus, and always keep the novelty to a level that does not intimidate your dog. If you're socializing your ball-crazed dog to tall men with beards, ask such a man to play a few rounds of fetch with him. If you're socializing the dog to car rides, take him on short trips to fun new locations and try not to drive him anywhere unpleasant. (If you do have to drive him somewhere un-fun, such as to the vet's office, try to stop a ways from your ultimate destination, play a few quick games in the parking lot, and then walk to the bad place.) If you're socializing him to potentially scary equipment such as a wheelchair or walker, get an empty one, sprinkle a few treats around it, and let the dog investigate them at his own pace. Later, progress to a slowly moving wheelchair, then one with a person inside.
If possible, try to have your foster dog not just see the novel stimulus, but interact with it in a pleasant, positive way. If the stimulus is a person, this generally means having the person hand-feed the dog treats (unless your dog is too nervous to accept them from the person's hand, in which case the treats can be tossed a short distance). If it's an object, this usually means letting the dog thoroughly sniff the object, play with it (if appropriate), and/or find treats hidden in or around it. Never force your foster pup into any interaction -- the goal is not to scare him! -- and always respect any indicators of stress he shows. If a dog is seriously uneasy around small children or visibly predatory toward birds, that's useful information for the adopters.
Here's a non-exhaustive list of things that a foster dog should, ideally, be socialized to before moving on to his forever home:
- Cars (both riding in them and seeing them pass by)
- People carrying umbrellas, wearing sunglasses, in bulky coats, and wearing funny hats
- People of different ages and ethnicities
- People using mobility aids
- Children under 8 years
- Children under 2 years
- Men (many dogs are wary of or hostile toward men, because male humans tend to be larger and more imposing than female humans; usually this is due to undersocialization rather than a history of being abused by men, although it is often misattributed to the latter)
- Teenagers (many dogs are wary of teenagers, who frequently travel in groups and, boys especially, with a swaggering gait that dogs can find threatening)
- Meeting other dogs on leash
- Playing with other dogs off leash (if safe and appropriate)
- Cats and small animals
- Grooming (teeth brushed/examined, feet examined, toenails clipped, fur brushed, ears checked)
- Gentle restraint (being held immobile briefly, possibly in conjunction with grooming exercises)
- Being hugged around the chest and shoulders (many people like to do this; many dogs find it unpleasant or alarming; do your best to make your foster pup think it's great rather than terrifying when a well-meaning person tries to hug him)
- Walking on different surfaces
This list is, of course, incomplete. The more of the world you can show to your foster pup, the more relaxed and resilient he'll be in his new life. It may not be possible to hit the entire list (I've never managed it, despite my best efforts), but do whatever you can. You cannot overdo socialization.