Adair,When you cut your groove in the deadwood, try to make it bigger on the inside. So the when the whip thickens, it locks itself in. Like of like a dovetail joint.
Also, shave the whip on the side where it will contact the deadwood. That way, the whip won't build new wood on that side. It will build new wood on the outside. When the whip builds new wood on the side next to the deadwood, it tends to push itself away from it.
The deadwood should be a very hard wood, of course.
And finally, try to make it so that you can see where the live portion comes out of the soil. If it's all on the back, it looks more "fake".
I have always wanted to try a different technique. Once the inside wood has been shaved, how about application of a super glue to hold it tight into the deadwood ( maybe let the shaved wood dryout a bit first)?
I wondered if this would work better than attaching with screws and less evident.
What do you think? It may be worth the experiment.