Please do not contact me saying they don't work because they do if you do it right.
 Just doing everything in the right order isn't enough, timing is very important.

Resetting The Service Indicator

Switch on the ignition to position 1.
Select maintenance indicator within 4 sec by double tapping the instrument cluster button.
Switch of the ignition to position 0 (off) within 10 sec.
Push and hold in the instrument cluster button and switch ignition on.
Hold the instrument cluster button for another 10sec.
In that time, the spanner symbol on the LCD will flash and eventually reset.

New Smart 451

It is the same as the other smarts, as listed above.

Important Info

Some people have said that having the doors closed helps, I'm not sure about that one.
Please remember to get your car serviced properly, resetting the light doesn't
mean your car has been serviced and you won't fool anybody when you come to sell it.

I have only posted it here because there are a few unscrupulous rip off
attempting to sell this info on EBay. Don't fund these cretins.