On the high score table type one off the following:
"R.J.TOONE"--------For infinite lives
"BUMPER"---------For infinite bumpers
"PUSSYCAT"-------For nine extra lives
"BARMY CAR"--For an extra 1000 points
"WOOAARRGGH"---------For faster turns
"WHOOPSIE"-------------To skip levels
"ARNIECAR"-------100,000 bonus points
Also, if you want to stop press the 'Z' key for brakes, and to get extra letters leave a single dot at the end of the level to make one appear.