Add plenty of well rotted organic material

I would until recently have also suggested adding grit but the RHS suggests huge amounts are needed

http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=620

That's a really helpful site, Anchorman (of the similar avatar). I wish I'd noticed it before. So now I'm going through a mental list of the plants already in the garden - yes, yes, no!, yes etc. Not doing too badly so far.......

You could add grit locally in around your planting holes along with your humus stuff though. I'm trying this under a sedum planted last autumn.

Best thing is to fork over the area adding lots of bark and other bulky organics into the area. I also add lots of gritty compost. That seems to work well for me.

Raised beds are a quick way to plant plants which struggle in compacted clay soils. They also drain quicker. You still need to fork over the area as all you will be doing is putting some compost on top of compacted clay and no root is going to penetrate that and water can accumulate underneath causing root rot.

I would avoid adding anything but the grittiest sand as that adds nothing and does not help drainage and in summer all you will have made is concrete.

Sometimes adding calcium, in the form of limestone or gypsum, helps too.