Drive a more fuel efficient car. And it's better for the environment too! Let me give a personal example. I went from driving a 1996 Dodge Avenger to a 2008 Toyota Yaris.

1996 Avenger: 21 mpg (rated @ 24 mpg, but it's old)
2008 Toyota Yaris: 32 mpg

With gas at $3.50/gallon and driving 12K miles a year it would cost me:

1996 Avenger: $1750
2008 Yaris: $1312 (save $438/yr)

Let me give some comparisons:

2008 Toyota Prius: $903
2008 Ford Escape Hybrid SUV: $1312
2008 Dodge Grand Caravan: $2100
2008 Ford Taurus: $2100

If gas were at $4.00 then you would save relative to a Grand Caravan/Ford Taurus:

Prius: $1368/yr
Civic: $1230/yr
Escape Hybrid: $900/yr

**Here I am just giving fuel savings for a fixed price and driving behavior. This is not a "cost to drive" value. Cost to drive also depends on capital cost (vehicle purchase price), maintenance costs, depreciation and car financing. Here are the cost to drive numbers for my:

Avenger: $0.25 / mile (assuming $2.50 gas)
Avenger: $0.30 / mile (assuming $3.50 gas) Yaris (so far): $0.78 / mile (assuming $3.50 gas)
Yaris (12 yr): $0.26 / mile (assuming $3.50 gas)
Yaris (15 yr): $0.23 / mile (assuming $3.50 gas)

So this is what has happened since the last entry:

1. Refinance My Mortgage from 6.75% 30yr fixed to 5.5% 15yr fixed. Cost me $700 in fees. But will save me close to $80,000 in the long run. My payment went up $150, but I can afford that.

2. Bought a new car with almost $4000 down. As of today I have $10,400 on the loan with a monthly payment of a little over $300.

3. Got my new car rear-ended. $1800 in damage. Got my car fixed, looks brand new again. Cost me a whole lot of time and $50.

4. Bought a bike for $325. Will save me about $1 everytime I ride my bike to work. It will take me a few trips to earn it back.