Another method to increase organic matter in clay soils is with cover crops. These are crops planted as you prepare beds, for a season or year prior, or in fallow periods between annual crops such as flowers and vegetables. They consist of small grains and grasses like buckwheat, ryegrass, and oats. Legumes, such as clover, also benefit by fixing nitrogen from the air for use in the soil. Cover crops have additional benefits such as suppression of many weeds.
If all this sounds like too much trouble, till or break up with a spading fork the area you'll plant. Then build a berm, or raised bed with sides 6 to 12 inches high or more, over it and fill with a good loam topsoil. You can use drainage tiles, or pipes, on the bottom if a wet area.
If you're planting an individual tree or shrub, it is especially important to chose ones that will tolerate clay soils. Their root systems are so extensive that you can't amend the soil well enough over a large enough area, especially for trees. Dig the hole only as deep as the pot or rootball. Otherwise it will sink over time, causing the plants to end up too low. This is a major cause of woody plant demise after a few years.
Don't make flat sides to the planting hole that wont let water drain. You'll only be creating a bathtub for the roots, without a drain. And only amend the backfill soil with no more than one third of a better soil as the plants were growing in.
If planting large plants, break up an area around where you'll plant (out to as wide as the plants will eventually grow) with a long spade or fork. Add organic matter on top of this area which, over time, will work into these cracks you made.