While the golf swing can certainly exacerbate lower-back issues, especially if you've been playing a lot, the real culprit to any soreness you might be feeling down there typically stems from the hours when you're not on the course. In short, you're sitting too much.
Sitting down for prolonged periods can lead to a host of problems for the soft tissue and bones at the bottom third of your spine. A big issue is what I like to call "glute naps." The massive muscles on the posterior side of your pelvis (your butt) are supposed to support your spinal column when you sit. But they can easily become inactive if you're not paying attention. Instead of sleeping, you need to put these guys to work.
Strengthening your glutes with exercises like lunges and deadlifts, or even simply walking up stairs, will go a long way to reducing or eliminating lower-back discomfort. But that's not enough if you're still sitting for the majority of your day. The problems will reoccur unless you do something about it. So what should you do? Try this awesome stretch you can perform anywhere, including at your cubicle right this minute. Click on the video to see me demonstrate it.