The way I've always done it is to sheet the main in until the boat begins to power up; cleat it; then have it loosely draped (like in between your thunb and forefinger) on your forward hand, which is also used to steady yourself by holding onto the tap handle.

Your aft hand steers while all of this is going on, but you shouldn't have a death grip on the tiller. Your hand has to slide along the length of the tiller until you are fully extended on the wire - without steering the boat.

Once you're out, you cinch in the last bit of mainsheet, holding it in your forward hand.

Or, if you want to be real fancy, give the main to the crew and you work the traveller