Here are the do-it-yourself links. Somewhere in there is a more detailed description of what to use to keep the upper spring seat from turning.
The 2 that I can remember are wedging the head of a 17mm wrench between the nearest bearing tower and the upper spring seat on one of the Hex flats so it will not turn and the other was inserting big Screwdriver between the Valve Spring Coils and the uppper seat to put more tension on it (I am otherwise not sure of the details on that).
I would think anything that would hold it with out causing damage would work (Channel Locks?).
http://www.peachparts.com/shopforum/forumdisplay.php?f=82
http://www.peachparts.com/Wikka/DoItYourSelf