I motivate my students during summer months just by building them up for how fast they are learning, setting the bar high but attainable, making up FUN ways to study, offering them cool beyond cool (but free) rewards for achieving high standards, and then challenging them to master new material before they see me next. For example, I taught a student a fun way to keep memorizing flash cards whenever he gets tired of studying: he walks around the house with a deck of memory cards. When he gets a card wrong he puts it back on the bottom of his deck. If he gets it right (and very quickly) he just flings the card back over his shoulder to land ANYwhere, with the attitude, "yeah, I smashed that one... it's HISTORY!" My student LOVED this and his family got a kick out of watching him do it. I set the bar VERY high for him to master what we're studying, and offered him a tour of a fighter jet (he LOVES jets) if he masters it all this month. His mom gives me email feedback like: "what did you do with my son? This kid is an ALIEN! I've never seen him so fired up to study!"