Make them watch replays of their own games.
(I commented on Eric's answer, which I like, but I thought of several points I think add up to a separate answer)
I suggest you get them to watch replays of their own games. A great guerrilla way to do this is for you to very explicitly announce that you're going to watch the replay of the last game to see what YOU did wrong, then go watch the replay. You're 1) giving them a little peer pressure to watch it and 2) giving them dead time to fill before the next game starts, so they have nothing better to do. You'll also get better by watching the replays yourself!
If they're not making MASSIVE mistakes, (never building anything that can defend against air) then don't tell them what they did wrong at first, unless they ask. Let them figure out their own mistakes if they're at all inclined to do so. They'll naturally notice and learn things they were closer to learning, whereas your suggestions might be far from their current awareness. Also, anyone who watches a replay and WANTS to get better will naturally watch a battle they lost and try to figure out why they lost it, without you having to do anything! If they don't do this naturally, there may be little more you can do for them, because it likely means they don't really WANT to learn.
If they're doing this, sooner or later they probably will ask you why they lost a battle that they don't understand. Definitely answer, but in concrete terms they can emulate. "Better micro" is abstract and hard to duplicate. Still say it, so they get used to the term, but focus more on "I killed your air units first" or "I used my unit's special abilities". Those are simple ideas they can understand and try themselves. The abstract things like "Better micro" are doing all the little things together, faster, which will only be learned through practice anyway.
Whether or not they're asking questions, you should be complementing them on the things they got RIGHT as you watch the replay, especially (again) the concrete things they can do again. "Scout more" is abstract, but "that marine you sent out found my unguarded expansion right when I didn't want you to" is easy to repeat. You'll plant the seed that says "send out a lone Marine now and then" and they will.
And even if you can't get them to watch replays, even just talking about the last game for a few mins can help. You can do the same things... compliment the good tactics, answer questions. And overall, always try to pose things in a positive way. "I thought you were going to X and destroy me" is an opportunity, but "you lost because you didn't X" is a criticism.