What's the best way to increase my horsepower on my 2012 Mini Cooper S Clubman ?

 

Aftermarket exhaust and air intake.

 

Also, don't forget to get it tuned after you do these. I am new to the Mini Cooper land, but I have other cars with a turbo on them and swapping the air intake and exhaust without getting the car tuned for those items can decrease the value of them. So I always recommend getting the car tuned in order to make it safe and get the most out of your upgrades.