I am not a speech therapist, but I have a son who has been receiving speech therapy for 6 months (he's now nearly 2). I can share with you things that his therapist and I have done that have helped.
I want to praise you for your willingness to support and supplement what the speech therapist is doing. Of course, as a former teacher you understand the great need to work as a team member to the benefit of your son. Your son will be better off because of your hard work and involvement, so kudos to you!
Ask the therapist for homework! We did that from session one. We also had as many sessions as we could at home (both my husband and I work full-time, so often she had to hold the sessions at his daycare), so we could observe the things she was doing with him. She would tell us words to practice, things to do, etc.
Label things - not with words, but by naming objects as you hand them to him. At meal times "cup" when you hand him the cup, "spoon" when you hand him the spoon, "puppy" when you hand him the stuffed puppy. If he makes any sound remotely close to the actual word, praise him and repeat it. Talk, talk, talk to him and keep labeling things (articles of clothing "pants. we're now putting on your pants." body parts "nose. where's your nose? there's your nose!" food "cookie. yummy yummy cookie.")
Start making him ask for things. This is a technique therapists call 'sabotage'; you have the cup of milk or the cookie ready. In order for him to get it, he has to either say "more" or sign it (tap fingers together). The food becomes a motivation for him to speak. Of course, this technique shouldn't be used when he's really, really hungry nor should he have to 'ask' for his food all the time. We would give him a few bites of food, then we would role play. Daddy would start making 'unh unh unh' sounds to indicate that he wanted a teddy graham and I would look at him and say "more?" and he would either sign/say the work 'more'. When he did it, I gave him the teddy graham. Our son was tapping his fingers and saying 'more' almost right away!
Animals - if you have flashcards or puzzles or books of animals, name the animals and give them a sound "puppy - woof woof" "cow - moo". Start only with a few at a time, and then show the picture and say "where's the puppy?" or "what does the puppy say?".
I have many more tips, tricks, and homework assignments that we had. I'll try to think of them and add more! Good luck to you!