Make sure you have a solid edge along the entire run of the flooring. Get some 1by2or4s 4' long. Lay a few rows of flooring and then wedge it tight with a 1by2 tacked down to the floor. Then move on down continuing the process of a few rows at a time.
When everything is dry and solid, remove the tack strips and work your way across the room doing a few more rows at a time with a tack strip every 4' to hold them tight. Don't forget to stagger your joints.