It often is a good idea to start with a service. Replacing the oil and
all three filters gives it thicker oil that will not get past the
piston rings and valve guides as easily. Changing the air and fuel
filters removes a slight strangle on the supply of fuel and air and it
all burns a lot cleaner.

Once you notice it running a lot smoother you can take it out for a
good thrashing and blow all the accumulated muck out. It's best to do
this at night when there aren't any following cars, unless you are the
kind that doesn't feel embarassed at leaving a smoke screen . Once you
have the engine up to normal running temp accellerate hard keeping it
in as low a gear as possible.

The place that does my MOT says they get some cars that have been
treated gently and produce great black plumes when they do the smoke
test.