Definitely lift up your hands and keep the connection between your horse's mouth and your hands, collecting his energy. You tend to lose that connection. If you keep his energy focused between your leg and the bit, he'll have to be engaged and paying attention.
Try going around the edge of your ring or wherever it is that you're practicing, in a wiiiiiiiiiiiiide circle. Let the tension go out of one side of your reins, tighten up with the other side (not pulling, just connected. There's a difference.) then after 3 strides switch to the other rein tight. This is more of an exercise for your leg than your hands, actually, despite appearances, to get him engaged from your leg.
To help bring your hands up, one of my favorite exercises is to take a crop and hold it in both hands, horizontal across the neck, so you can't bring your hands down because the neck is in the way. You can't collect much like this, I've found, or at least I'm not very good at it, but it's a good exercise and it gets the point across.
If you look around the internet/in books, you'll find a ton of more exercises to help keep your horse collected and as a consequence energetic from the leg. These are just my favorites.
If you feel like you're disunited coming to a jump and you're just jumping small by yourself, don't be afraid to just circle before the fence to give yourself more time to get that good, engaged canter going.
If you like flatwork, have you ever considered riding dressage? Just a thought.
Hope this helps!