* Pro tip: choose or make up an obscure department that no-one ever has any business with. Statistics or internal auditing is good.
Wow that exactly what I was going to say and what I actually do! I actually am a 'civil servant' now and work in an obscure department (R&D and Innovation) and my job does include an element of statistical analysis and auditing data so I normally waffle on about this aspect of my job (ignoring anything interesting or useful) until people move away.
I learnt along time ago to never tell people I was an IT Consultant or else I ended up being expected to solve every bodies computer issues.
One trick I used to use when self employed was to keep business cards on me all the time and when anyone asked me an IT question in a social situation I'd whip out a card write a ridiculous hourly rate on the back and told them to call me in the morning.
This led of course to one of two situations, I never heard from them again or they called me and had to pay a slightly silly amount of money for help/advice. I was equally happy with either outcome.