I don't know how common such things are around the country, but we have clay, although the soil reports we have gotten back from MSU over the years all call it "sandy loam" so go figure, but I believe that the best results that we have had was with Canadian peat. But that comes with a cost, and manure has worked well too. I prefer cow manure, because the subsequent weeds seem fewer, but horse manure is definitely easier to come by. And I do agree too that gardening rules are made to be broken. We have a master gardener down the road who we are good friends with, and several times she has said something like this or that won't work, naturally after we have already done it, only to return later to get more details on the affected flowers or vegetables that are thriving.