The lights on stands are usually the main culprit of extra heat in our plant spaces, so to help combat this I stagger the timing on the three separate stands. No more than two stands are lit at any one time. However, you can also employ a nighttime lighting routine, only having the plant stands lit during evening hours. If I didn't have a/c, this is what I would do, but I need to have the plant room lit during the day so that I can get my regular chores done. Although quite a pain to do, another option is to split up your stands so that they are in different rooms. For those with long, hot summers, this could work well.
During the earlier parts of summer, when daytime temps begin creeping upward while nighttime temps are still fairly low, one big problem is powdery mildew, especially during humid weather. Circulate the air in your plant room, 24 hours a day, with a fan. I use a standing oscillating fan and find that I have to change the height of the fan periodically so that all the shelves get good air circulation. It's common to have plants on lower shelves develop powdery mildew. To combat PM, I use Funginex at the first sign. I've not noticed any negative side effects to using it, and it's very effective. Lysol can also be used, but it tends to mar leaves (since the aerosol is fairly cold,) and it can also kill flowers.
One more big problem in summer is that our plants dry out much faster. A watering routine that worked well in winter won't be as effective in summer, when temperatures are so much higher. With the warmth comes more productive plant growth and drier air. The plants will utilize much more water during the summer, so it's imperative to monitor your plants closely and adjust your watering schedule accordingly. If plants dry out too often during the heat, their leaves will pale and droop, sometimes taking a long time to recover. Also, avoid the tendency to fertilize and/or over water when they are dry, since this can easily lead to root rot and burning.
Keep yourself and your plants cool this summer! More next time.