I don't teach or recommend so-called "purely positive" methods that allow misbehaving pups to continue misbehaving, instead of teaching them which behaviors are and are not allowed. "Purely positive" is fine for teaching tricks and high-level competition exercises, but NOT for teaching the solid good behaviors that all family dogs need to know, and NOT for stopping behavior problems such as barking, jumping, chewing, nipping, chasing, etc.
Like small children, puppies have short attention spans and get tired quickly. Keep your puppy training sessions short – 5 to 10 minutes is enough – but regular. Two or three short training sessions every day is ideal. And, remember, you can build in the ‘come’ command throughout the day; for example, when you want to feed your puppy or play with him.