I first lower my ticket prices a bit and see which parts of the stadium fill up. At the early stage I tried to expand the bleachers and lower tier, it's good value for money and great for generating lots of revenue at the start. The downside of those seats is that the attendance tends to vary each game and if you build too many and they won't fill up, so only do about 10,000 bleachers while in div 3.
But I'm now concentrating on courtside and luxury boxes, even though they're expensive, they eventually pay themselves off and will generally fill up almost entirely.