Takes practice. If I'm going out on the wire for the first time, usually I'll unlock the hotstick and let it extend as I stand back onto the wire, so I never move my hand on the stick. If I'm coming out of a tack, generally the stick would already be extended, so just slide my hand along it as I stand out. I find that I am constantly making small adjustments to the hotstick length as I'm sailing. I prefer to have the very end of the tiller in the palm of my hand, so if I step forward or back or make any other slight change of position, I will adjust the length.
The most important thing is to be looking at where you're going and looking at the wind coming down on you. If you are looking down at your tiller, mainsheet, or hands, your steering will be all over the place. Also make the transition on or off the wire as quick as possible. Trust the wire and harness to support you. Many people new to the trap have a lot of trouble with this and want to hang on the trap handle or have other issues trusting the harness to support them. If you're having trouble with trapping in general, get comfortable trapping from the crew position before transitioning to skippering from the wire.