Don't thrash it!
Ooops sorry!
Seriously - not only don't thrash it, but;
don't floor the throttle, even at low revs, just "chase" the revs with the pedal,
don't use too low a gear (but equally don't be in too high a gear & have to use large throttle openings to get going)
don't use the brake unecessarily - wastes speed you're only going to have to get back later.
I know this sounds dull, but a lifetime spent commuting along boring traffic filled roads teaches you a few things about how to spend your precious fuel / money.
PS as I have a Cooper, I haven't voted, but I'm regularly over 42mpg - even kept around 40mpg on a run with Becca & Matt on Monday around country roads