You definitely have enough seats after your next expansion. First of all you should lower the prices of your luxury boxes, the bleachers too, so that they are almost sold out. This would be the perfect situation.
Very important to get more customers is that you win the away game before the up coming home game. This makes a huge difference in attendance.
As already mentioned a winning streak would be great, but quiet impossible if you're only 3rd-5th best team in your division.