Wet Track Handicapping Adjustments

[wigmore]

I am at a total loss in handicapping for a wet track (or later, a muddy track).

I have friends who say choose the frontrunners, because they aren't getting water/mud kicked in their faces. Makes sense, but it doesn't seem to work for me. I have another friend who says I should key the horses who have already proven themselves on an off track. This strategy has resulted in limited success.

Should I limit my bets when I find myself at the track when it is raining or has recently rained? Or can someone give me a few clues in how to approach handicapping an off track?