Fuel additives are on the whole rubbish but the oil additives aren't.

They can reduce engine ware and improve fuel efficiency. The type of thing you see advertised on these half hour infomercials on late night TV.

One of the best things to do is make sure that your tyres are in good shape and at the correct pressure. You can have the best car in the world but the bit that dose all of the "work" is the tyres they are the only thing in contact with the road...
(check out the tracking, it's a good idea that those tyres are facing the right way)