With the "being used as a tool" thing, try to think less about the people "using" you and more on your own thoughts.
Personally, I don't care whether other people are using me or not.  If it's something I like, I'll do it, if I don't like it, I won't.  Simplifies things down a lot, and it makes yourself the centre of everything.
Yes, this is a bit self-centred, but if you feel others are taking advantage of you (in other words, they're being self-centred), try doing a bit of it yourself.  It also allows you to be more frank with people - it's difficult to say to someone not to use you as a tool.

Also remember that they don't have usually more "bargaining power" or "control" over you.  You appear to be rejecting people who seem to be trying to sweeten their end of the deal, but you can also try to level out the field and perhaps take advantage of them.