This is a well discussed question but I'll give you a summary. Make sure you're tires have the correct air pressure....less resistance with the road and the engine has to work less. Use synthetic lubricants...again less resistance. Of course there are the mechanical mods like cold air intake to increase air flow into the engine. A high flow exhaust to match the increased air going in with the crap going out is also needed. On top of that a performance chip will help. The last step is a programmer to make sure the new bits and pieces are working nicely together. You can do any of these steps separately but for the best results all the steps needed to be packaged together.

I'm sure others will have more suggestions...spark plugs, plug wires etc etc.