If I had one tip.....IDK. Really I have several.
Like said above. Get one, practice with it, and don't be scared to use it. You have to use it enough to learn it, get comfortable with it, and give it a chance to work. Closed reed is easiest to learn with. Don't over call and try too hard. A nice moderate toned cottontail will kill a lot of coyotes.
The sound does not have near as much to do with success as the other 90% of calling predators. First, find an area that has plenty of predators with NICE FRESH EARS. I really can't stress this enough. There have to be predators there, and they have to be call-able and not highly educated. Really pay attention to your sets. Work hard on good setups.
If you are successful with an e-caller you will be with a hand call as well. If you aren't successful with an e-caller, a hand call is not a magic bullet that will change that. Even though, I do find that moderate/minimal calling with a hand call can sometimes net some animals that blasting an e-caller won't, just due to it being a totally different style sound than they are used to.
What I have said might sound brutal or condescending. I promise you, I do not mean it that way. Not at all. What I mean is that the majority of the time your location and setup is going to have way more to do with your success than the sound you use. There are exceptions to that, but it is a good general rule.
As far as blowing a mouth call I try and put myself in the shoes of the rabbit or animal I am trying to mimic. Realize they have small lungs. Put some hurt and emotion into it. Let them "get tired" or excited, or whatever. Experiment with the call, and how to make it do different things. Watch some videos, listen to tapes. How to use your lungs, mouth, and hands in concert together to change sounds, tones, cadence. Try and mimic what you hear, and then don't be scared to try something else.