Socializing
Socialize, socialize, socialize.
I live with a fearful/fear-aggressive mutt monster who, owing in large part to a severely undersocialized puppyhood, would still be unable to live safely in the great majority of adoptive homes despite years of intensive remedial work and behavioral medications. So let me say that again, loud and clear: 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 life 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.