Ditto on the magic box, except mine is on the deck. A line runs from the outhall sheave, through the boom, down through the reef cleat that I retro-fitted with a small cheek block, down to a block on the mast partner and back to the box. It's expensive, but lighter than you'd think and it allows another control line to be within arms-reach of the trimmer.

:: My boat has one of those Harken magic boxes in the end
:: of the boom for an outhaul, and amazingly it works
:: great. Furthermore, it's very easy to extract by just
:: removing some screws.