First and foremost, let's state the most obvious effect of summer flying: it. is. hot.
Being a pilot from Texas, I can personally say you should check on your southern friends. There's a good chance we're dehydrated and .2 seconds away from passing out due to heat exhaustion.
Okay, maybe a little overdramatic....but it is hot.
When flying in the summer, whether as a student, flight instructor, or any type of general aviation pilot we need to understand the effects of the weather changes.
Rule number 1: Always carry water. Even if you just hydrated before your flight and don't think you need it, grab water anyways (and by the way, try to go green and make it a reusable water bottle while you're at it). From first-hand experience, dehydration and heat exhaustion can have a bigger impact on flying than you'd think. Your decision-making skills and effectiveness on hand-flying the plane start to deteriorate. If ignored, dizziness and a headache can start to occur. This becomes even more important on long haul flights. Don't be the newest accident statistic due to poor flight preparation. Even if you're in a rush, take 2 extra minutes and grab that water.
Rule number 2: Take into account the changes it can have on aircraft performance. If you're taking off from an airport with a short runway, even if a ground roll is normally adequate, double check it. The hotter it is, the longer ground roll you need. That point you're used to rotating at or obstacle you're used to clearing might not be your friend today, especially as the heat rises continuing into August. A great tool to help gauge the temperatures at the surface and at altitude is the GlobalAir Aviation Weather Temperatures Tool. Just click the "national weather" tab, then click "temperature" and see it all illustrated on an analysis chart. A quick tip: if you're using it to plan a flight for later in the future (and not 30 minutes from now) make sure you click for the right time frame!
Rule number 3: Still on the weather subject, check your winds before heading out. You're most likely to encounter gusts of wind on a hot summer day with calm winds at the surface. I've also experienced this firsthand, so it became a learning experience. As soon as I reached 1500 ft the wind picked up, and it didn't stop. The turbulence actually reached moderate for me that day, so I cut my flight short and went back. No sense in taking chances to keep going and fighting the plane the entire time! For any situation with undesirable weather or even maintenance issues remember this: it's better to be on the ground wishing you were in the air than in the air wishing you were on the ground. The GlobalAir Winds Aloft Tool is also a great resource in planning for this. Be sure to check this and local METAR/TAF for each upcoming flight to ensure you don't get in a situation making you wish you were on the ground.