Getting into Summer Seminar will help you a great deal, assuming you're not a screw off when you get there. There are more applications to seminar and less avalible spots than the actual plebe class. More than one person in my squad during seminar applied and attended with absolutely no intention of attending USNA. Apparently it looks really good on other applications as well, who knew... Of the ten or so people that I met and kept in contact with from seminar, none who decided to apply were denied, one went to prep, but mostly for USNA's unique way of "redshirting" football players.
Note: All this information is pretty dated, especially application numbers.