For planting, incorporating lots of organic matter and a little sand or gypsium will make a big difference. For hardscape, remember that the dry, hard clay will become very plastic soft clay at some point when it rains a lot. Therefore, more base prep is needed for hardscape projects. We deal with clay on a regular basis. I normally put down a geo-textile fabric to separate the clay from the base crushed stone. Increase the depth of your base by at least 2 inches. If you are putting up a tall wall, (over 4') have a soils engineer test the soil for strength and make recommendations. Your client should pay his fees. Oh, last point, pray that it doesn't rain because clay takes forever to dry out in cold weather.