In reply to Owain: Currently working on an article on this..but here's one we did earlier..
http://www.thebmc.co.uk/Feature.aspx?id=2329
Technique wise there isn't that much to learn. Going down hill 'breaks off brains out' is the basic tip. Your upper body is a throttle, lean in you speed up, lean out you slow.
Don't get fustrated walking, it's normal. we all do it.
Re shoes, depends, trail starts at good forestry trails, which road shoes are fine and then blurs into fell where more pure fell shoes are suited. Without knowing what sort of terrain you run it's impossible to give too much advice. I'd go for a more generic trail shoe at first, so the TR3 willbe fine for a wide range of terrain.
Do's: Run with others, enter races, be adventurous try different routes. Enjoy.
don't: Get fustrated if you have to walk some sections of hills.