Use two nails at each intersection of the decking board and the joist. Roughly 1" from the edge of the board works great.
Don't forget this step. You will need the nails every so often to guide you when you nail the rest of the decking.
Every so often, measure from your starting point to make sure you're laying the decking square.
To correct any issues, squeeze the decking a little tighter on the longer side.