What a worry for you.
I consider myself to be pretty good at marketing and advertising, but despite my best efforts I'm much quieter this year than last. It's great in one respect because I can use the time on the distribution side of things, but on the other it's the salon that pays our mortgage, so I need to keep busy!
There are a few thing you can do I think. You can alter the opening hours of the salon a little. So if you're open 6 days a week you could squeeze all your clients into 5 days.
One of my friends has just told her staff that they can either drop a day or be made redundant. It's a tough choice but if she doesn't do it she'll go out of business, and her staff need to know that. By dropping a day you'll save electricity etc, and it'll make the remaining days busy and vibrant. You could use the day off to do networking, marketing, admin, planning, training etc.
You could also reduce the hours each day - if you're open 9 - 6 you could make it 10 - 5 on some days and have just one late night. Another friend has just shifted her hours on a couple. She used to be open 10 till 6 and now she's open 11 - 7 and she's picking up the after work people she used to have to turn away.
With regard to the other girl, it's too easy to be rocked by such unscrupulous people when it's quiet. Every time you worry that it's her you're giving her more power. Concentrate on thinking positively. If you haven't read The Secret, now's the time!
Remember that you're in business to stay in business, not to go under. Do whatever it is you need to do to protect it. If that means letting people go, or making changes to your hours, then do it. Things WILL pick up, but huge sacrifices will have to be made until they do.