Never take off solo in a two-seater without the aft cockpit checked and its cushions and belts secured. Another essential part of pre-flight inspection for a two-seat sailplane is weight and balance. If you need ballast, there is no excuse for leaving it on the ground. When one occupant is unusually heavy, placing that person in the aft seat will keep the center of gravity closer to mid-range.
Flying dual with another pilot, whatever the rating or experience of either of you, and regardless of your personal relationship as friends or otherwise, never signal for takeoff, or approach for a landing, before establishing who has technical responsibility as pilot in command, and more importantly, who has control of the aircraft!