Hi, the old fashioned way was to double dig the border, incorporating well rotted manure, garden compost, horticultural grit/sharp sand etc, into the bottom spit, and then adding the same materials into the top spit annually, mulching annually and regular digging, but this method seems to have been superseded in favour of a much less labour intensive way, which is to just spread the above materials onto the top spit, and turn them in, or some people just let it work it's own way into the soil, but I prefer to do it the old fashioned way by double digging, I made a new bed a couple of years ago using this method, and when I tested the drainage, it was quite well drained, [I dug a hole about 18" deep and filled it with water, and it drained away in 4 1/2 minute], as we're into autumn you can use whichever method you choose, but instead of breaking up each spadeful, leave it in large lumps and let the frost break it down as well, then dig again in early spring, adding the aforementioned ingredients. Derek

I am on heavy clay, wet in winter, solid rock in summer! I don't dig, too old, too lazy and arthritic! So the garden gets piled with as much compost, wood chip and even grass cuttings in depth and the lovely little worms sort it out eventually. Obviously the best way is above as Derek says, but not all of us are able.

Compost, absolutely. Lots of compost.
You can also use gypsum, calcium sulphate, to help reduce the stickiness of the clay particles. Unlike lime, which also can help, gypsum will not increase the soil pH.

I used a product called Claybreaker, which I think was essentially gypsum, but found it only partially successful. Elsewhere, umpteen years of digging and growing things have gradually had the desired effect. I hope that doesn't sound too discouraging.