> On the matter of her secret identity as Karen Starr, has it ever been established (at least in recent continuity) how she hides who she is?
> Clark (both of them) have/had glasses. PG's counterpart, Supergirl, used a change in hair color (at least pre-Crisis)...
> How does Power Girl make sure nobody recognizes who Karen Starr "really" is? Glasses AND hair color?
...unlike Clark, Karen Starr doesn't work in a field where she would be seen with her alter ego. Clark is a reporter that works to report on stuff that happens to Superman. Karen is a software designer. I think the reason why she never took on more of a disguise is because there would be no reason to link her to Power Girl, visually, whereas Clark Kent would have his picture in a by-line next to a picture of Superman.
And I'm sure Power Girl, as owner of a software company, probably dressed more conservatively. Probably just doing that would diffuse suspicion. But you can probably add to the idea that many women were probably emulating Power Girls' "look" at the time (new, young, feminist blonde, busty superheroine with a paige-boy cut), and you have a reason why Karen Starr looks kind of like Power Girl.
...she also got her identity after she was fighting with the JSA for a while, so that someone who looks like Power Girl could be chalked up as a "wannabe" rather than "oh my god, that's Power Girl."