I use a plastic triangle (about 5 inch sides) made out of an old car number plate with jib hanks screwed to each corner, two corners on the jib halyard, one on the forestay. The device slides up as you hoist the jib. I got a drawing possibly off the Wanderer website. It is also pictured in the Wayfarer handbook, (with mention of the addition of a bit of shock cord to keep it right at the top, not sure how that bit fits).
I find it helps to slacken the halyard tension a bit before furling so that the corresponding increase in forestay tension helps keep them apart. Have to say it works fairly well but sometimes crew required to re-do it after adjusting halyard tension. cheers Dave