You are right to be concerned about posture, as an incorrect body stance while playing the cajon could cause soreness or fatigue, and you definitely don’t want a repetitive strain injury. Rest your feet flat on the floor, and keep your back straight as you play. (Slouching is not healthy for you, and it won’t help your sound, either.) Spread your knees so you can easily access the head, keeping your buttocks centered on the seat. You might need to lean slightly at times, but the majority of your playing time should not be spent leaning. Playing the cajon should not be painful. If you develop soreness or other problems, have a friend watch you while you play and offer suggestions, or sit in front of a full-length mirror to evaluate your posture and make corrections.