An Ounce of Prevention
This section overlaps considerably with Training and Troubleshooting, but I'm categorizing these exercises as Socialization games because they involve habituating a foster dog to routine occurrences rather than teaching specific actions in response to cues. Done correctly, they should hopefully prevent a dog from manifesting troublesome behaviors rather than requiring you (or the adopters) to break a bad habit later. An ounce of prevention is worth a pound of cure, and so here we are.
Absence Makes The Heart Grow Stronger
Every once in a while, beginning on the first day of your foster dog's arrival and continuing for as long as you have her, leave. Don't make a big deal of it, don't say anything, just walk out of her sight and ensure she can't follow. Go into another room and close the door, walk away while she's in her crate eating, or leave to run an errand and return.
You don't have to be gone long. Five to ten seconds is plenty the first few times (particularly since you probably won't want to leave your foster dog unsupervised that long anyway); you can gradually work up to longer durations. Ideally, your foster pup won't even realize at first that you've disappeared, especially if she's busy gnawing on a new rawhide or eating a meal. If she does realize it, and starts crying, don't go back until she gets quiet.
The purpose of this exercise is to accustom your foster pup to the idea that sometimes you will go away, but you will always return. It's essentially an emotional inoculation against separation anxiety; it teaches her that she can be alone for short periods of time and the world won't come to an end. In fact, it's so benign that sometimes she doesn't even notice she's alone until it's over.
Food Bowl Exercises
There are several ways to do these exercises. They all aim toward the same end: proofing against resource guarding by teaching your foster pup that it is not a bad thing when you approach while he's eating, reach for his food bowl, or even take his dinner away.
Version 1: Divide the dog's meals into several portions. As he finishes each portion, take his food bowl away and put the next portion in. This shows him that your taking away the bowl predicts more food, not less food, and is therefore a good thing. (If your foster pup has any resource guarding tendencies, wait until he's thoroughly finished and is starting to lose interest in the bowl before you attempt to remove it.)
Version 2: Walk past the dog's food bowl while he's eating and nonchalantly drop in an extra-special treat: a chunk of cheese or hot dog, a scoop of canned food, or whatever else he really loves. This teaches him that a person approaching his food bowl is not a threat to be scared away, but a predictor that his dinner is about to get even better.
Version 3: Take the dog's food bowl away while he's still eating, drop in an extra-special treat, and hand it back. (Do NOT do this one if your foster dog already has resource-guarding tendencies; this exercise should only be done with dogs who don't seem to mind being hassled while they eat.)
Version 4: Hold the dog's food bowl while he eats, and/or stroke him gently and babble quiet praise throughout his meal.
Play is a very powerful reward for most dogs. You can use this to your advantage in several ways. One of those ways is discouraging resource guarding by playing a lot of games that require your dog to give up the precious toy in order to keep the fun going.
Retrieves require the dog to return the ball (or Frisbee, or whatever you're throwing) to you in order to get another throw that she can chase.
Find It! is a game wherein you hide the toy and ask the dog to find it and bring it back to you for a reward. I use Kongs for this game because it's very easy to teach a dog that finding an empty Kong and bringing it to you will be rewarded by your putting food inside the Kong (which he then has to get out, keeping the dog entertained even longer). You can combine this game with Stay practice and improve his impulse control by putting your foster dog in a Stay while you go hide the toy.
Tug can be used both to teach bite inhibition/mouth control (any touch of the dog's teeth on your hand ends the game instantly) and to reduce the likelihood of resource guarding. Let the dog "win" the game periodically by dropping the tug toy. Wait for him to bring the toy back to you before the game resumes.
All these games teach the dog that he only gets to have fun and/or a treat if he gives the toy to you. This is a powerful message. Additionally, games burn off a lot of doggy energy, which is great in its own right, and improve the human/canine bond as you have fun together and you-the-person are seen as a source of delightful entertainment. So play!