I think the only way to reduce the gas prices with a long term effect is to reduce the use of gas as prices are usually influenced by
1) the available amount
on which you don't have a lot of influence, it is controlled by the oil industry
2) the requested amount which many of us can reduce if we want, e.g.
- ride your bike,
- walk,
- use public transport,
- if you need to drive a car use one that needs less gas
- if you need to drive a car drive in a way that needs less gas
- ...

This will have more influence on gas prices than just not buying any gas on one particular day - the gas will still be bought, before or after this day. The only effect might be that the people working at gas stations have a lazy day!
Andreas