Theres some info here:
http://www.pistonhea...ectomy - do it!
Basically there is a big piece of foam in the centre of the backrest which can be removed (theres more foam behind it).
You can then take the base cushion out, and cut about an inch off the bottom of it (again there will be loads of foam left)
People refer to it as a 'foamectomy' and theres a few links on google. I did it to the drivers seat on my mk1 mx5 and it made it much more buckety and very comfortable.
The covers are held on with metal rings - cut these off with snips, and when you put the covers back on, use cable ties in the places where the metal rings were