I would plant trees on the downhill side of your bed, that way the tree can get to the water easily. Plant things around the top of the mound that don't mind dry periods as it will dry out first. Once the bed has some ground cover on it and shades the earth, your bed will stay at a more constant moisture level. There are no real "rules" in the end, just try whatever ideas you come up with, try them. Hope that was of a little help.