What an awesome topic! I completely get what you mean. We seem to be programmed to agree to favours, not stand up for ourselves etc. For years I was just so eager to please and so terrified of doing the wrong thing that I was a complete pushover, bullied all through high school, everything.
Strangely enough becoming a lawyer helped me to no end. Suddenly I had a duty to a client not to be a pushover, and to stand up for myself (ie, them). Once I had to behave differently for a third party, it seemed to give me the confidence to stand up for myself in other areas of my life.
I think it's about just stopping and thinking 'is this situation reasonable?' and if not, saying no.
As for the in laws, can't apply it to that. I just can't speak up for myself there. I don't know why. I think I just hate the thought of making life difficult for DH so I try to suck it up (and then have a mini breakdown that DH has to deal with anyway when we get home).