If you have a DSLR and you don't have a bag then that's #1. It escapes me why you wouldn't but anyway.
It sounds as though you take your lens off your body when you're done? That's unnecessary. The more times you change your lenses the more often you're exposing the mirror to dust. Dust travels. When you change your lens in the field I would point your camera downward, ensure you have your lenses ready for a quick swap (loosen the rear lens cap). If you're in conditions where there is wind or other weather issues then take cover and change the lens or try to work with what you have on if you're just shooting casually.
You can always invest in a Rocket Blower, this is by far the most used tool in my bag. It blows air, not compressed air (that's bad), and you should start the practice of using it each time you start shooting and then finish shooting. Think of it like getting ready to leave and then washing up once you get home. Your camera needs a little wash too (don't use water ok).
I am unsure about the zoom BS the guy told you. The lenses you use should be sealed pretty darn tight and the lens on the body should create a fairly tight seal depending upon the camera you have. It's usually a twist and lock motion so I'm not sure how much of a vacuum this creates. It sounds silly but I've been known to be wrong often.
Finally, you want to make sure that you take it in for a good cleaning or learn to do it yourself once a year. I firmly believe with the Rocket Blower you'll get by with an annual cleaning if you find a dust bunny has just taken residence on your censor. I would not recommend cleaning it yourself until you've learned all the various ways to do it wrong then the one way to do it right.
