Naturally, I had my feet checked out a while back and I have a tendency to supination (i.e. walk on the outside of my feet) on the left hand side.
Clever old people at the running shop I visited started me off with a pair of Asics shoes worth £110. They were like walking on air, but out of my price range so I tried some cheaper shoes. Well - no surprises - but the Asics ones were by far the best so I've opted for them. I still have a pain in my left foot from the training I've done but these help significantly.
I've also been warned that I will probably lose my toenails. Nice!