There was a thread about this on either this forum or the woodland forum. You can dig a shallow trench to slow down the encroachment of lawn grass. You will have to beat back the grass once in a while with almost any system. Sometimes I dig a shallow trench and lay logs in it to make an edge/border. Laying the log in the shallow trench keeps it from rolling around. Some people use bricks or pavers flush with the soil surface. You can also buy flexible metal, wood or plastic edging. You will also want to heavily mulch a few inches inside the edges of your flower garden. Sounds like a great project you are working on.

You might need to soak the clay and wait a few days before you work it. I don't think I'd work clay soil that was either too wet or dry. Personally, I think stones would be the nicest, and of course the longest lasting.

--Ron

I mostly use just a shovel cut along the edge of the lawn with perhaps 4 to 6 inches of scraped soil, then the garden. This looks nice and neat when I first make it or after I have just spruced it up, and it allows me a place to run the wheels of the mower so i can mow right to the edge of the lawn without cutting anything in the garden. TO do this I simply take a standard garden shovel and cut the edge of the sod to make a nice edge. After repeating a bunch of times I have a neat edge for the lawn. The lack of any edging material makes it really easy to repeat the procedure if grass spreads or weeds grow. ANy time I have put stones, logs, etc. for an edge I get weeds in around the material and it becomes a pain to have it there. The plain dirt method is a lot easier to maintain, and I think looks pretty nice if kept neat.