I am not so skilled with a board like that and going upwind. But my trick sounds to be very close to what Roger says...
In spite of using a pretty short board, the theory could be similar.
I try to remain close the the mastfoot, the back foot only so far that my nose remains above the water. I go a little bit on the windward side. If you now apply the tack technique the board will go upwing very fast (loosing speed very fast). What I do is not to put tha sail back so radical, I keep it more upright. To win the speed and not loose it I am sheeting in when the sail is back and sheeting out as I move the sail forward...this is like pumping, but you move the the sail back and forward in the same time.
May looks funny on the water but the upwind performance in non-plannnig conditions is fine.
Wish you a good luck.
PS: To the board for early planning...if you have enough patience and you do not want to buy another board soon take something else than Go 155. It may take a bit longer till you get it running (full speed, foot straps etc.), but it will be than also a lot more fun...