it's been standing in the garage / outside under the cover for years - getting turned over once a month... once every few months...
or just twice a year including Christmas!
After a few years the darn thing won't start

Dotty was running very rough 3 months ago when last checked over. It gave me a sad feeling as the engine is (was) fully rebuilt and it hasn't even driven anywhere yet.
I came to start it again at the weekend and she refused to play! Not even pretending that it might start. It had a healthy spark and some petrol in the tank and even a healthy tick from the petrol pump.
Wasted over an hour on it on Sunday - checked and double checked the electrics. Petrol was definately getting through as the pump would run during/after cranking, and the plugs were getting wet after an hour of cranking.
Came back to it today with some fresh willpower but the same result.
Then splashed some fresh petrol in the mouth of the carb before cranking - it fired instantly!

So I drained the tank using the petrol pump. (only a pint)
A fresh gallon of petrol in the tank, flushed though the fuel line, reconnected the fuel hose (another splash into the carb).
It started on the button first time! and every time since then!
TIP:
If you think the petrol may be stale, hence the non starting - just give it some good stuff down the throat (before turning it over) to see what happens!
For safety's sake I wouldn't recommend splashing petrol about during cranking! - keep the can well away afterwards!!