I have my best results when using the electronic and mouth calls in tandem. We usually set up with electronics first to bring birds in, especially if we are hunting "dumb" birds. The trick is to keep a check on the volume. Once you hear crows talking back to you, turn the volume down. Extremely loud calls will spook birds, especially if they have been shot at before.
After the first shots are fired I'll usually kill the electronics and switch to a mouth call and make a crow in distress call. That's usually good for one more shooting session even on educated crows, especially if your set-up/camo is right. I can remember one set-up in particular in which we called birds back for six different shoots in the same set-up. If you have the right amount of emotion in your mouth calling the birds will come. They are so curious they just can't help it.
The biggest mistakes we made starting out was not taking set-up/camo seriously enough and turning up the call too loud. I'm torn on decoys. For "dumb" birds they can be a help in bringing birds close. I think educated birds are best hunted without decoys. Seems like they recognize the lack of movement and stay too high to shoot. Seems like they get along more curious when they can't see anything obvious and swoop low for a closer look.
I'm not an expert, but I do consider myself pretty serious about crow hunting. It's probably my favorite past time. Hope this helps.