Well, I've worked in photo retail, though it was before these digital times: really, I don't think you should be 'pushing brands' at all: you should know what you're selling, know photography, and pay attention to the people who come looking for your advice. Their needs, wants, what they *like* when you put it in their hands, whether they are likely to want, say, more lenses or some such later, and their budget.
I always saw it more as matchmaking than some competition between brands, the latter of which people can go to the Net for all they want.
People are going to tend to want to ask, "What brand is the 'best'" and then try to select from among them: even if they don't know much about photography, that's kind of something they feel they *understand,* when really, each brand has its strengths and relative weaknesses for any given price range.
With Pentax, that can get a little interesting, since the K20d is generally compared to much more expensive models from other brands: it pretty much outclasses the Nikon and Canon of similar price ranges, though there are areas where the next step up, like a Nikon d300, has certain advantages, you've got to weigh those advantages and any relevant drawbacks against, 'Hey, you can have the Pentax and better *glass* than the kit lens, (And Pentax's kit lens is really by all accounts the best out there, but still a kit lens) which in the total analysis is really more important.
It's also best for a serious student to learn on, in my opinion, for the money, as it simply has the separate shutter and aperture controls, which the models costing the same as a K20d from other brands are bound to lack. (It's certainly most appealing for people who *do* know photography and have a tight budget: the simple fact is that the various Pentaxes are the only things that are going to work well with older lenses for less than a d300 costs: until just recently, with the Nikon you couldn't even get an affordable basic prime that would work well with anything less, (and good on them for coming out with one.) )
And you get the weather sealing and probably the best overall feel: those are some prominent strong points.
Really, you're in that store to serve customers, not brands, so what you need to do is help them make some informed decisions and get what they'll be happiest with in their hands: for different people there'll be different answers there.
(If the brand-non-recognition is scaring people you otherwise think might be best suited to a Pentax, well, put one of those classic old bodies on display, so people can see they've been around just as long as the more-advertised brands. Same might go if you carry Olympus. (Though I'd actually be pretty hard-pressed to want to sell someone an Olympus if they didn't know *why* they wanted it, already, as of the last time I looked at them. )
Oh, and Monochrome, it does seem that we've had quite a number of people coming to sow confusion (or just join in on it, ) on the News and Rumors forum in particular, though... why this question is in the Beginners forum maybe seems a little odd, come to think of it, but it's not like there's any harm in having the occasional stab at a question like this: hec, the general coolness and relative lack of snobbery about the global Pentax community could *be* a good selling point.
Buy a Pentax, kids, and you get to hang out with *us.*