For a few years I struggled as a full time FI and mostly enjoyed the job. Now I too (just like everyone else who has replied) am doing it part time.
Money was one aspect, I now earn easily well in excess of double I ever did 'FI-ing'and that is after things like pension/tax contributions etc have been taken off and. I now have far more job security than I did as an instructor. It seems even the best schools can do things on a bit of a shoe string, the worst ones just rip you off and I have been there too.
Just to add to the doom and gloom, a friend of mine turned down a well paid job to go instructing full time, within 2 weeks the school he was working for went tits up.
All in all instructing P/T has been so much better fun as I am now flying for love and not food.
On a positive note there are well paid/secure instructing jobs out there but they tend to be of the CAP509 type and you need CPL/ IR Multi etc etc.
As for flying Slingsbys for the mil as BCalm suggests I think that too requires CPL/IR plus lots of aero/formation experience (correct me if I am wrong), jobs for the boys ie ex RAF.
Because PPL instructing is so part time and not considered in the professional light it should be it is just knackering up the industry in the long run. A PPL instructor is worth at least £18-20000pa, isn't that what driving instructors get paid? They don't spend £30000 on getting their qualifications.