Treats are a cornerstone of any good positive training program. I suggest keeping several varieties on hand: dogs have different preferences, and it may take a few tries to figure out which are your foster mutt's favorites. Rotating treats keeps them from getting boring, and while treats aren't supposed to make up a huge percentage of your dog's diet, you will probably be feeding a fair number of them, so it helps to have a few different kinds of healthy treats to minimize the risk of your foster pup's diet getting too far skewed in any one direction.
Finding healthy treats is much the same as finding healthy food: look for simple, clearly identified meat sources (or cheese, peanut butter, etc.); avoid corn- or wheat-based treats and anything with a lot of byproducts or digests; avoid anything with artificial colors, sweeteners, or palatants. Treats that are brightly colored and pressed into gimmicky shapes tend to be of poor quality. There are a couple of exceptions, but most of the good treats are just plain-looking pellets or squares, and they're all relatively boring shades of yellow or brown.
It's a good idea to have a selection of high-value, medium-value, and low-value treats. Each plays a different role in training.
- High-Value Treats are the ones that dogs go crazy for. Small pieces of "people food" work best for most dogs: tiny pieces of cheese, chicken, roast beef, dry Texas-style barbecue, hot dogs, and so forth. Natural Balance makes a dog food roll, colloquially known as "doggie crack" among trainers, that you can also use as a high-value treat, although I would caution against using this treat heavily because sugar and/or molasses is very high on the ingredients list, and it's not really a great idea to feed your dog a big stick of sugar every day. Whatever you use, the pieces should be no bigger than the pink part of your pinky nail. These are the treats you use when teaching a brand-new behavior, asking your dog to focus in a new and distracting environment, or whenever you're raising the difficulty level and want to give the mutt a little extra incentive to work hard.
- Medium-Value Treats will probably be the mainstay of your training program. Most high-quality commercial treats fall into this category: Zuke's Mini Naturals, Cloud Star's Tricky Trainer treats, Blue Buffalo's Blue Bits, and so on. Larger treats such as Wellness WellBites and Pure Jerky treats, or Blue Buffalo's Wilderness Bites, can be cut up into quarters or sixths to make suitable training treats. (As is, they're too big and will fill up your dog too quickly, but they do make good Kong stuffers.) Choose moist, smelly treats: these are the most appealing to your dog. These are the treats you'll use for routine training sessions and fine-tuning behaviors your dog is already familiar with.
- Low-Value Treats include biscuits, dog cookies, and other dry treats. Evo Wild Cravings and Cloud Star Buddy Biscuits are good, as are K-9 Kraving Cookies (duck liver, bison liver, and green tripe). You can also use pieces of your dog's regular kibble as low-value treats. While these treats are too big to use as frequent reinforcers (except for the kibble), and most dogs are not willing to work very hard for them, they serve nicely as occasional "pop quiz" rewards. They are also crucial for training Leave It and Let's Make a Deal, where you want to exchange a higher-value treat for a low-value one. Finally, they're good to hand out when you're rewarding your dog for relaxation and don't want him to get too excited, as he might for the prospect of last night's barbecue. So even though you may not need a lot of them, it's good to keep some lower-value treats on hand.