First, and most important, never, and i repeat, NEVER leave the open camera body facing straight up. In that case, it is as if you were inviting dust to fall inside your camera. The best way to change the lens is to tilt the open DSLR body to face downwards while you are changing the lens.
Second, do not leave your DSLR camera body open for long. That also might be pushing your luck. You do not have to rush though, as you might (like me) drop the lens while rushing. Fortunately for me the lens ended up on my bed, but I learned this valuable lesson the cheap way. Second time i may not be so lucky.
Final tip, if you are in any type of extreme environment (high moist, sea spray, dusty and windy area), DO NOT CHANGE YOUR lens. I have learned this lesson during my trip to Thailand, where I was frantically changing lens on a long-tail boat trying to shoot majestic rock formations near Railey beach in Krabi, and a few drops of sea spray fell inside my camera body, fortunately on the mirror and the outer rim of the fitting. Although you might miss the shot, no shot is worth getting sea water on your DSLR camera’s sensor. If you are traveling frequently and do not want to miss any photo opportunities, you might be better off investing in a super-zoom lens (something like 18-250mm). They might not be the cheapest, but they are definitively the most flexible ones.
I hope you will find these information useful, and will not have to learn the hard way (as I almost did). Happy Shooting.