OP: pay your employees more. The gum chewing and other unprofessional behavior will magically disappear as your pay attracts better quality employees.

Here is a link to additional FREE resources on self-branding you may find helpful - http://strengtheningbrandamerica.com/tools/self-branding/

What your talking about is called executive presence, and it's something that has to be learned, especially for women.

I would talk to them one on one about it, as well as have a discussion with the team about it, or with your larger department about it. (In my company it gets discussed all the time.)

You're also talking about what's called the P.I.E. model:
http://strengtheningbrandamerica.com/blog/2012/01/the-p-i-e-model/

and how they develop their personal brand.

Whatever you decide to do, consider it part of their or the team's career development, and how it fits into the culture of your own workplace. There are also tons of courses and books on this topic, called personal branding.

I'm a 36 year old woman who has 4 employees reporting directly to me. Two of them have performance issues and are on documented improvement plans. The other two have good work performance overall but really struggle with projecting a professional image. They are both younger but not fresh out of college - employee A is 29 and employee B is 27. For various reasons this is their first professional job and they were hired at the same time about 2 years ago.

They are just minor issues on the face but I feel they are issues that can really hold young women back in the workplace. They both dress inappropriately - not in the sense of "clothes that would only be appropriate for the club" but they do dress VERY casually (we have a typical business casual work environment but they both are on the very casual end of that). During meetings they constantly fidget, chew gum, and doodle instead of taking notes. These are meetings with higher ups at our organization and the perfect time to try to make a good impression. Both of them talk like Kardashians with the vocal fry and "like" every other word, constantly adding on "or whatever," to the end of sentences, etc. The good news is that when they aren't interacting with others their work is good so their performance reviews have been good. I just feel that their overall lack of professionalism will hold them back from getting higher profile assignments.

I've tried to model good behavior but it's not getting through, and like I said I don't feel that it's a performance issue that could go on a person's review. Has anyone dealt with this in the workplace with younger employees and how did you help them to improve? I definitely would feel awkward about saying, hey maybe you could dress in non-wrinkled pants that aren't two sizes too big when the CEO comes in ...