I recently stumbled across this post from HBR on how to ask for favors: http://blogs.hbr.org/glickman/2011/01/asking-for-a-favor-the.html
Seems solid (and actionable!) on it's face. Anyone see pitfalls or have alternative techniques that have proved effected?
--Steve

Not bad, necessarily
For many, a "favor" is a chit, something that can be banked and cashed in later.
Asking for help, rather than a favor, is less transactional, and doesn't imply a quid pro quo.
John Hack
"Help" is in the eye of the beholder
I think however you phrase it, some people will take a request as a quid pro quo, and others won't. Prefacing your request with "can I ask you a favour?" is just acknowledging the situation and (as the article suggests) puts the recipient of the request in the right frame of mind. It's no different to "Can I give you some feedback?", in my opinion.