This was somewhat inspired by QT Luong's post. In other words, the best shots of the dunes are not on the dunes. In fact the location is quite accessible. I always get annoyed with obscure directions to photo locations though when GPS is so ubiquitous. Timing is everything, so there's no time to reposition.
In this case, proceed to 37° 41.240'N, 105° 33.213'W along CO-150 about an hour before sunset. Any later, and the shadows of the dunes may be too long.
Use a polarizer. This is very important for getting the right light reflecting off the sand dunes at this angle. You'll see the immense difference as you rotate the polarizer into position.
For this shot, I was using a 70-200/2.8, but I was shooting at f/8, so you don't need the 2.8ness; a cheaper, more travel-friendly f/4 or 5.6 should be fine. I was shooting zoomed all the way in to 200mm. I tried a 400mm lens, but that didn't allow me to get the mountains in the background. You could probably get away with a 300mm, but that wouldn't get you as much width in the frame. Obviously, I cropped some of the top and bottom of this photo a bit in post.
As you can see by the sand coming off the tops of the dunes, the wind was whippin'. It was pretty relentless too. I recommend using a very solid tripod. Normally people go for lighter tripods, but you'll be right along the road for this shot, so a heavy, solid tripod should actually work in your favor. If you can, use your vehicle as a wind block too. Even with a good tripod, I still had a few shots come out blurry due to camera shake; the lens hood is great at catching those gusts of wind.
The San Luis Valley is an amazing place, even without the dunes. I highly recommend a visit. Happy shooting!