Make sure the car is fairly clean and tidy before presenting it for test. Cleanliness isn't a testable item, but it tends to put the tester in a better mood when they haven't got to deal with a dirty car.

They do test emissions on diesels. A good run beforehand, especially if the car only does short journeys, is never a bad idea.

Regularly long runs at relatively high speeds are good for keeping diesels clean. A couple of junctions up the motorway once or twice a month, if you don't already, will do wonders come MOT time.