Dehydrated beef or chicken livers!

Made by companies like Beefeaters and Gimborn, these treats are lightweight, they contain no weird preservatives, and they're easy to break up into miniscule pieces. These are the food reinforcements I use when I train clients' dogs, no matter what skills or behavior modification we're after.
There are some important differences between food reinforcements and treats. Treats are things you give your dog because he's cute or because you love him. Treats are for fun. Food reinforcements are the paycheck your dog gets when he's learning something new. In order for a food reinforcement to be its most wonderfully effective, it has to meet a few criteria:
1- It has to be more delicious and impressive than any other food your dog gets, ever. Therefore, using a piece or two of his regular kibble doesn't really count.
2 - It has to be very small, about the size of a pea. Dogs are concrete thinkers who give new meaning to the term "living in the moment." If you ask your dog to sit for a particular hand signal, and he does, and you reward him with a big, chunky MilkBone biscuit, it is very possible, in fact, even likely, that by the time he's done chomping and swallowing and doing the "miniscule crumb scavenging" thing, he won't remember why he got the cookie in the first place. The payoff from the most effective food reinforcements should be instantaneous.
3 - Proper food reinforcements are used solely for training and conditioning. They are never "Freebies."