After you fit any performance upgrades follow these steps to reprogram your ECU:
Turn on the car and warm it up (make sure that it's idling normally)
Turn off the car and pull out your ECU fuse located in the fuse box under the bonnet (this is usually a 15 amp fuse found at the far left of the fuse box) If you can't find the fuse, you can always disconnect the battery. This isn't favoured by me, as you'll lose all your stereo presets!!
Keep fuse out for at least 5 to 10 minutes
Plug fuse back in
Start your car MAKING SURE NOT TO TOUCH THE ACCELERATOR WHEN YOU START IT UP
Let the car idle (at its normal idle) for about 5 minutes.
Shut off the car and restart it. Take it for a test drive and you should notice the difference.
That's it! Your ECU is originally programmed to mix optimal amounts of air and petrol for a factory spec car (no modifications). By taking out the fuse you kill all power to it, therefore resetting the 'program' that gives you the right mixture.
Now by NOT TOUCHING THE ACCELERATOR as you fire it back up, the ECU will reprogram itself to give you the best MPG possible based on your current set-up.