How do I enrich mostly clay soil?

Clay is hard to enrich and keep that way, Myself, I would check with a local greenhouse who knows your particular soil for what you will need to add to your clay soil. It can take a few years for it to start staying good. You can also dig holes for your plants that are wider and deeper than needed and use your potting soil to plant. The other way would be to have raised gardens and you don't have to worry about the clay soil or just use pots in your landscaping.

Is it in a garden bed,lawn? you don't really say. Add 1 bag potting soil & 1 bag organic compost dug to what would be about 2 bags of the clay down at least 8-10"s into the clay. Roto-till it all together would be easiest or you can use nicely sharpened(use a file) pointed shovel and your feet/legs to turn it under and mix it together.Clay is actually nutrient rich it just holds water and gets compacted to easily and that suffocates plants roots.It is best to improve an entire planting area all at once, rather than to attempt improving the soil in individual planting holes as you need them. If you dig a planting hole in clay soil, then drop in a plant and nicely amend only the soil you're using to backfill, your plant will be happy for a little while. But you have essentially done nothing more than create an in-ground flower pot. Eventually, the plant will start sending out roots that will be stopped in their tracks when they reach the clay walls of the planting hole. You'll end up with a root-bound plant that it won't grow as large or as healthy as it should.

Adding sand or perlite will help the soil drain better. I recommend getting soil samples done--ask your local Extension Agent. It is free.

Thank you for your response. I am going do just that. Our local extension office is at the other end of our county. It will be worth the trip.

I would not add sand to clay soil. Can create a harder, concrete like mix. Gypsum is supposed to help, and incorporate lots of organic matter. Takes a long time to improve clay soil.