The cost of fuel
If you live within cycling distance of your work place try riding to work. Just once. One commute by bike is usually all it will take to prove to yourself that it's pretty easy. My commute is 7km and it takes just over 15 minutes. At peak hour in the morning it can take 40 mins by car, more if you include the wait for a bus (then walk to the office) or taxi. The make it a goal to cycle more than you drive, taxi or bus to work. The only problem I've found is having a shower. If you have a good boss (or are the boss) find out how much it will cost to install a shower. You could even offer to contribute out of all the money you save on fuel (yeh right!). If there is no shower at work arrange to have one at the local pool, or find someone who has a shower in their office.
I've saved over 1 tonne of green house emissions and if saving the environment is not incentive enough how about the $300+ saved in fuel... about $80 per month.