For long term heat-proofing your home, air sealing is one of the most cost effective measures. Most energy loss does not occur through windows or doors. Even if the attic is properly insulated, if there are air leaks then hot air is infiltrating into the living area.
Many local utilities will do a blower door/infrared camera test on your home. When I did this, the "aha" moment was seeing that my kitchen walls were reading 100 degrees F. The reason was that the interior walls were open to the attic at the top of the wall, and hot air was circulating inside my walls. This made the kitchen extremely hot in the summer.
I hired a contractor to seal the air leaks as identified by the IR imagery, and the leakage of my house was reduced by 33%. My house now holds a more constant comfortable temperature. The next step was adding insulation, but this should only be done once the air leaks are sealed. Adding insulation to a leaky house does not stop the leaks. My city rebated about 40% of the cost of this work (it cost about $1700 combined).
Un-closed chimneys, dryer vents, and fan vents all leak energy. Try to seal your chimney when not in use, and install one-way dampers on other vents where possible. It makes a huge difference.
I live in a climate where it can reach 100F during the day, but it cools to 60-65 at night. I use a whole house fan at night to cool the interior down very cold, then shut all the windows in the morning. Last summer I went the entire season without needing A/C. I recommend AirScape fans because they are quiet, small, easy to install, and efficient (just a customer).
Don't think that just because your home is new that it is not leaking energy. Our local utility audited the leakiness of many homes and found that the most leaky ones were built in 1999. Before spending five digits to replace windows or upgrade your A/C, get your house energy audited. Otherwise you could be wasting money.