Oil prices have fallen to less than $100 a barrel and gas has declined to an average of $3.85 a gallon, down from $145 a barrel and $4.05 a gallon two months ago. Perhaps you reduced your time spent in the car.

But faced with some breathing room, you now may be tempted to relax your mileage-conscious ways. Instead, look at lower fuel prices as an opportunity to wring out a household budget surplus.

Say you used to drive about 15,000 miles a year (1,250 miles a month) in a car that gets 20 mpg. Once gas prices rose, you saved $81 a month if you cut out a third of your driving -- down to 850 miles a month.

If gasoline prices continue to fall, you could save even more by maintaining your miserly ways. When refineries come back online, gasoline prices may again track the falling price of crude, and could end up closer to early 2008's $3.20 a gallon when oil was last around $95 a barrel. If that happens, you could save an extra $36 a month by continuing those initial mileage reductions.

Now, $36 a month isn't a huge windfall and likely won't make up for other economic hardships. The bigger reason to keep the budgetary belt tight is that, even if gasoline prices drop down to the $3.20 level, they are unlikely to stay there for long.

"The No. 1 reason we've seen the price of oil and gas come down is because of the overall problems in the U.S. and worldwide economic slowdowns," says Geoff Sundstrom, a fuel price analyst with

AAA

. "As the economies recover, we'll likely see a return of higher energy prices."

A growing economy means more construction, more manufacturing, more trips to the mall and more travel in general -- all of which require energy. And while high prices brought about lots of talk about energy plans and conservation, all of that pain will be for nothing if we forget the lessons learned and return to the status quo.

So if you now bike to work, take public transportation, telecommute one day a week, drive more conservatively, stay home more or drive a more fuel-efficient car, your best bet is to make it a permanent change. That way you can pocket the money now that prices are lower and be better positioned to weather the bad times when prices head back up.

Peter McDougall is a freelance writer who lives in Freeport, Maine, with his wife and their dog.