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+1, be friendly, it usually works.
The only hostility I've encountered was some geriatric bloke on a green lane (unmetalled road) telling my I shouldn't be riding there (it's a public road technically). I politely corrected him, and his wife apologised for his ignorance!
Also got shouted at for riding across a golf course, on a bridleway. Once again, I stopped and corrected the guy, and he shut up.
^^^ All good points re: access, dealing with other trail users.
Here's an example from one of my local trail advocacy groups:
http://www.peakdistrictmtb.org.uk/index.php/81-you-re-just-a-nuisance
I ride plenty of footpaths, cheeky trails etc., employ a smile and a cheery hello if I ever meet other trail users, and have yet to encounter any hostility.