I do not know where you live and how much gas costs there, but in Europe I would highly recommend a lpg-conversion for your (petrol) car. In Germany, you can nearly cut fuel cost in half... Depending on your car's design you might loose trunk space, though.
What car do you have? Perhaps it might even make sense to buy a commuter car. Early VW TDI engines (1.9) can not be praised enough, they are ridiculously fuel efficient, annoyingly loud and will last forever. (Beware of the 2.0 TDI, they are prone to injector and cylinder head trouble). Being rather old, they can be had rather cheap - I have seen Passats trading for less than 1000. Granted, with high mileage and some major repairs expected to come (front axle is a bit tricky), but still a great price per kilometre.
But that depends on ful price and used car prices in your area, you have to do the math.
Carpooling, on the other hand can not be overestimated. Cars are designed to fit 4 people, just driving them alone is absolute waste.
Personal example:
I study in Berlin, but my wife lifes in Munich (good job, and not likely to change - unemployment rate in Munich is 2%, in Berlin over 10%, and wages in Berlin are much lower...), so I have a lot of 600-km trips to make. Carpooling pays for my petrol, leaving me with only taxes, insurance and repairs as car costs (no depreciation, as I always drive old cars in the 1000-5000 range...). All cost included, a kilometre costs about 20 cent (the lowest I ever got) to 30 cent (very decadent v8-powered Mercedes S-class). My current cars are around 25c/km, so a one-way drive cost 150. (In reality, a little bit less, as taxes and insurance are sunk costs I would have to pay even if I did not drive this weekend). That roughly equals a train ticket, but trains would take 2 hours more. However, with carpooling every co-traveller pays me 35. Thats 105 for three of them. 150-105=45.
So with carpooling I can cut commuting expenses by two thirds!