To hit longer drives, you have to get aggressive. And more importantly, you have to get aggressive in the right way. In the example here, let's look at the first move you ought to make as you descend from the top of your swing and into the golf ball. The longer hitters on Tour (just about all of them these days) know that the greatest way to generate power is to leverage the upper body against the ground. This means, the first move they make as they initiate the downswing is to squat into the ground and push the legs against it. Specifically, you want to apply more force into your left leg than the right to aid in making a better pivot onto your forward side on the downswing. In the photo here, you can see how my head is lower at the onset of my downswing, as compared to the very top of my backswing. This force both downward and into my left leg, allows my upper body to build torque and, as a result, make a faster, more aggressive rotation through the hit.
A great way to practice this is to make a series of practice swings where you take the club to the top of your backswing, and pump your leg into the ground. You should feel more weight applied to your left leg than to your right one. As you do this, it's likely your arms will automatically start pumping downward too, indicating you're on the right track toward creating a powerful strike into the golf ball. After a few tries, hit some balls, and I bet you'll start hitting longer more aggressive shots immediately.
Why do we wear spikes on our golf shoes? One, we don't want to slip and fall walking all over the golf course. And two, we need traction and stability as we make golf swings. That isn't to say the foot shouldn't move during the swing–it does, but without traction, we would lose sight of how the foot should move.
So what do you do? Get a good pair of golf shoes. Pay attention to not only spike patterns for traction (almost all shoes have good traction now), but also what's called good torsional stability, meaning they don't twist like a common sneaker would. A good rule of thumb: The less your shoe twists, the more you can twist your body and make a fuller swing. Second, look for shoes that have dual pod sole configurations. This means the toe region and heel region of the shoes are actually separate pieces of the sole. This helps you better shift your weight from the back of your foot toward the front–again, a must-do if you're looking for more distance in your swing. And finally, don't neglect comfort. Just because a shoe is made with less twisting capabilities, doesn't mean it can't still be comfortable to wear and walk in. Look for shoes with adequate cushioning and shoes that lace up securely. Most blisters come not from stiff or new shoes, but from shoes that don't fit right. Follow these guidelines, and your feet will thank you.