Make it so a character can only effectively don a disguise or set their appearance in rooms with installed mirrors. They can still remove a disguise or clear their appearance anywhere. They can also don disguise items anywhere but they would not actually disguise you unless in a room with a mirror.
This would mean that you would need to plan the use of disguises and appear more carefully and that it is much harder for someone with no or little investment in disguise to maintain that disguise while out and about than it would be for someone who really put a lot into the skill and relevant stats.
The desired result is to see characters donning their disguises and making appearance changes then going out knowing that, if they have little or no disguise ability, they are on the clock and that changing out disguises and appearances will take real planning and is not something that can easily be done on the go.
As it is someone with no or little investment in disguise and relevant stats can essentially remain disguised forever. They simply get a feel for what they can do before the disguise fails, track what they've been doing and will then remove and re-wear disguise items when it's 'about that time.' This means that the pressure to increase one's disguise skill and supporting stats is less than I feel would be ideal.
I have also seen what I believe to be characters who are making drastic changes to their appearance both rapidly and on the go. Start on one end of the street as a hooded curvy mona and end up a hooded tall mano at the other after a brief dip into a quiet parking lot. Or shifting gender, height and build rapidly like they would change clothes. I personally feel that appearance changes should take forethought and effort to apply and not be used willy-nilly.
I know that some would suggest that requiring an installed mirror is unrealistic and I agree to some extent. But I find it no more unrealistic than the idea that people can don disguises and change their entire look while on some quiet rooftop on in a busy bar.
While equally unrealistic I feel limiting where a character can effectively don disguises and change appearances would be better for balance and encourage investment in the disguise skill and related stats and encourage people to be more mindful in their use of disguises/appearances instead of them just being always on and rapidly flipped through. I also feel that the idea that you need to see yourself and have both hands available (thus no hand mirrors) is an easy enough pill to swallow without breaking immersion.