a google search for topping lift will give you all kinds of ideas... type it into the browser and then click on "images" a couple inches down on the left side of the browser window...
there are a couple different setups... one is rigged almost identical to the main halyard, and connected to the aft end of the boom. the working end of the line can be cleated at the mast, cabin top, the end of the boom or at the foot of the backstay.... this line can be made to serve other purposes as well...
the other is a dedicated topping lift that has a small 3/32 wire (or whatever size you want) coming down from the chicken head and stopping about 4ft short of the aft end of the boom with a small block hanging from it....then a 10ft piece of 3/16 or 1/4 sta-set is tied to the boom, ran up and thru the block, and back down the the boom, and tied off....
you can be as creative as you want when attaching the line to the boom... with either a knot or cleat, but an eye strap, a small cheek block and a small cam cleat come to mind.
use the eye strap for the termination point of the line, and mount the cheek block close to the end of the boom on the opposite side from the eye strap...... as the line returns from the upper block, run it thru this cheek block to turn it forward on the boom, and mount the small cam-cleat where its convenient for you to run the topping lift from.... then you can easily adjust it as you desire.
personally, I like and use the dedicated method with the wire as there is less stretch in the line and the boom is solid when its on the lift.
I used to have the line going up and back down the mast for a topping lift, but there was too much stretch in the line for my taste and the boom always felt a bit spongy.... maybe some super low stretch line would have helped, but this current setup was more affordable and seemed like a better idea..... and it has proven to be for my needs