Talk yourself into it
People often underestimate the power of the mind. I remember an interesting experiment with which a speaker on women's empowerment used to open all her presentations.
Hold out your arm at 90 degrees and hold it there. Then have someone try to push it down and they'll have some difficulty.
Now repeat out loud: I am weak and worthless, a failure with no power. Say this five times. Try holding the arm out again and it will drop easily with only a little push.
The point of this is that your muscles (i.e. brain) reacts to the words it hears even if your conscious mind doesn't believe them.
I have used this form of self-hypnosis in reverse many times to convince myself that I was up to various challenges over the years, and even though I felt the tasks were beyond me, hearing the words told me I could do them, and I did.
Of interest to your own life, is a new study about the Placebo Effect in which a number of hotel maids and nurses who were overweight, were given a new perspective on slimming.
Each of the participants led an active and physically demanding life, yet didn't have time to go to the gym. Subsequently, they felt that they didn't do any exercise and thus they just got fatter (is that politically correct?).
When it was clearly explained to some of the participants that they were indeed getting all the exercise they needed during the course of a normal working day, they promptly lost weight.
This means you can now simply think yourself thin after the Christmas, while sitting on the couch eating Jaffa cakes and watching Eastenders (or Mythbusters as my demographic suggests).
Wonder if this works for building rock hard abs and pecs too (see above, it does).