I would contact an attorney. As for the cash flow problems, if someone can't pay you in a timely manner I wouldn't mess with them. We have bills to pay and we aren't the bank, which essentially is what they are using you for. I don't know what else needs to be said. Too bad she works for your competitor and not you.
Liens are a good idea.
Like another said, check with your county for their guidelines, and then put it your contract what will happen if payment is not made. Depending on the amount owed, you can take them to small claims court and they have to pay all your expenses and interest. It happens to everyone. Be aware of people or companies who do not stay with one contractor or who have employee problems. One tactic that works for collection is embarrassment.