when im overpowered i do two things: get my ass as fast into the footstraps as humanly possible so i get in a good position. I lean in with all my weight: legs slightly bend so you're a bit more seated then you should (it doesn't give the extra speed but it ads to the controle) and then i strart pulling the lot upwind so the power in the sail becomes bareble and i can start streching my legs and take up a good position. And yeah: move the harness lines back and also, but the farther appart, when i'm seriously overpowered i'm talking two fists between the two ends. This will divide the weight more across the boom which gives you more controle and tires your arms less. But these techniques all have one major dissadvantage: top speed is limited. You wanne go fast? start working out.
Haiko, AKA crazychemical