A fellow PhD candidate at Monash Uni (Australia) is researching in the area of Habit Theory and soft drink consumption. This might be of interest, if so then I could get a few reference articles for you or pass on your contact details (he is always looking for collaboration opportunities).
Thanks for your kind answer! Still, I am looking for an alcohol-specific measure, preferrably tested and validated. I have tested the Readiness to Change Questionnaire and the Readiness Ruler, but would need someting more concise.
Maybe this would help? http://pubs.niaaa.nih.gov/publications/AssessingAlcohol/InstrumentPDFs/62_SOCRATE.pdf
And the actual measure: http://casaa.unm.edu/inst/SOCRATESv8.pdf
Maybe this would be helpful as well: http://www.harmreductionjournal.com/content/3/1/3
One instrument which also has promise is URICA (the University of Rhode Island Change Assessment). For some information see below:
pubs.niaaa.nih.gov/.../ InstrumentPDFs/75_URICA.pdf
www.uri.edu/research/cprc/Measures/urica.htm
Animals or humans? Progressive ratio schedule of reinforcement is an often-used way to measure motivation in animals. In case of habit, you should use a reward devaluation prior to test.
You could try using the Treatment Motivation Questionnaire (Ryan, Plant 1995) It was normed on a community sample of adult drinkers and divides motivation into several domains; internal motivation, external motivation, confidence and help seeking