i have done favours before and it has backfired also,after doing a few kitchens for a local company the owner has me to tile his hearth and internal porch which i charged him pennies for,a few jobs later he started getting really cheeky with me and narrowly avoided me wrapping a spirit level round his fat baldy head,even his mate the plumber was horrified by the way he was trying to treat me so i tend to stay away from that sort of scratch your back scenario,however if someone is getting me a fair bit of work i tend to pass on wee gifts such as bottles of whiskey(my mate is a manager of bowmore and i get £30/40 bottles of 18 year olds for about £12)