I take public transit only when I'm carrying something too big to carry on my bike or when I need to be dressed more formally than is feasible on the bike - generally, not more than about three to four rides a month, usually all on the same day - I save up "heavy errands" and run them all at once with an unlimited daily metrocard.

As for the outrageous cost of keeping a car (as Dahon.Steve mentions), even indulging my bike habit pretty heavily, I'm saving a lot of money (and burning a lot of calories) commuting by bike - by my figures, keeping a paid-off Honda in New York cost me an average of close to $420 every month for the two years I did so; $180 went to parking and $193 to insurance every single month, and I averaged a break-in every nine months or so with a $500 deductable. So even with my current obsessive bike-upgrading and an unlimited metrocard every now and again, my wallet is happy

-chris