Well you can always delete all of the lights and lower the quality and size of your render.

This will make for a faster render, but it may not look very good...

As always, generally it's a balance between speed and quality.

Transparency in things such as hair or glass will also push up your render times.

If you are using iRay, you don't necessarily have to wait till it finishes rendering completely. If you are happy with the image quality just hit cancel on the main DS screen (Not the render window) and save it out.

Luxrender won't give you a faster render either quite the opposite from my experience.