Go to the Mosquito webpage and read up on they write on soloing the spi.

These guys have advanced most on this solo issue :

http://home.vicnet.net.au/~mosquito/

or directly to the how to steer and pull-retreive the kite topic :

http://home.vicnet.net.au/~mosquito/sp_sailing.htm

These guys seem to prefer to keep the tiller between the back of their lower legs and buttocks and steer by rolling on their feet with both hands free for the spinnaker.

See picture :



Wouter