DXM doesn't strike me as a drug you could get away with in public very easily. The obvious harm reduction advice is to not take drugs in circumstances where you're required to conceal it.
Since you're probably going to do that anyway though, you should at least try to be on a more appropriate substance. I've never had a problem getting away with being moderately stoned in public. I find that AL-LAD doesn't dilate my pupils and leaves me very functional. I once spent all day doing cocaine before work; that night my boss pulled me aside and complimented how hard I had worked that day.
I'm not trying to suggest you do any of those, just pointing out that some substances are more suited to the task than others. Dissociatives would probably be the last class of drugs I would choose.
