"What sort of corrections do you use?"
The harshest correction a dog should EVER receive is a time out.
This can be a bit misleading.
The harshest correction is what the dog considers harsh. If I smacked my dogs on the butt they'd think I was inviting them to play, but for one of my dogs breaking eye contact and turning my body away is the end of the world for him. The dog determines the reward and the dog determines the punishment.
However.... *Ideally* the dog does not get punished at all b/c he is set up for success, not failure. Many trainers believe in setting a dog (especially a dog with "issues") up to make the mistake that got him at the trainer's to begin with. They set the dog up and then punish for making the mistake that they knew he was going to make anyway.
Then some trainers just aren't experienced enough and push a dog beyond what he's capable of and he makes a mistake that has to be corrected.
Personally I find it much more effective to set the dog up to succeed and not allow him to practice the undesired behavior to begin with.
The better the trainer, the less need for punishment. Every time I've had to punish my guys it has been because of a mistake on my part. The more I learn, the less I find myself in a position where I have to punish period.
Now watch, now that I've said that, someone is going to start a squabble and I'm going to have to beat 'em all off each other