Well, I'm no master but after trying it out for a little while, I've learnt of some things that seem to work well for slaves.
Stop them for cumming for days
The idea of chastity and not being allowed to cum without your permission tends to make slaves very submissive and willing to please their master. Remember, until their master is satisfied, they're not allowed to cum.
Make them think about punishment
If you constantly remind them about the consequences of misbehaving, they'll be less likely to defy you. Also if severely punish your slaves for doing the smallest things, they'll know not to cross the line again.
Make them "prepare things"
If you tell your slave to get a number of nasty things without telling them why, you can make them start imagining horrible tasks. The things you make them prepare can really be as nasty as you want because (unless you're feeling cruel), your slave isn't actually going to need them. You can also combine this with the previous point to make them behave like a good little slave.
Specifically control when they cum
Right about when you're going to let your slave cum, don't just give them permission. Ideally, force them to cum a few seconds after you tell them. One good way to do this is to make them cum after you count them down but it's particularly fun to make this countdown last minutes, not seconds.