You can twist yourself in knots trying to dodge a lawsuit, but it really comes down to the avoiding the Big Six Mistakes.
Whitney Warner, an employment attorney with the firm of Moody & Warner, says these are the problems that come marching into her office most often: 
Not giving a reason for firing. You’re an at-will employer, so you can fire at will, right? Wrong. Most employees think they’re wonderful workers, and if they get fired for a mysterious reason, they’ll make up their own reason – or their lawyer will. The reason for termination needs to be clear.
Firing an employee for bad performance when the employee has good performance reviews. This is the cousin to “not giving a reason for firing.” Supervisors need to understand that they’ll need a poor-performance paper trail if they want to fire someone. Or else a judge will smell something fishy.
Poor timing. (a) An employee files an internal complaint about the employer or a supervisor, and then (b) shortly after is disciplined for a supposedly unrelated event.  It won’t be hard for a lawyer to connect the dots in court between (a) and (b). Employees who file complaints can be disciplined, but the supervisor better have the documentation in order before making the move.
Delayed internal investigations. When employees file complaints, they want them thoroughly investigated and they want it done now. If you can’t investigate immediately (because, for instance, a key player is on vacation), let the complaining employee know why and when the investigation is likely to begin.
Improper response to an EEOC charge. If you’re contacted by the Equal Employment Opportunity Commission regarding an employee complaint, respond promptly and courteously – and treat the complaining employee courteously, too. If you’re tardy in your response or treat the employee like a leper, expect to hear about it in court.
Failing to follow your own policies. You can have the best policies and training in the world – and indeed some companies have used that as a defense against a complaint. But you better be able to show that your supervisors followed those policies and applied the training.
Bonus: How to lose a lawsuit
Getting sued is bad enough, but after a lawsuit is filed, employers can make the situation worse: Being unprepared for depositions, an inability to locate key documents and responding “I don’t remember” to questions about key events will sink you every time. All of that makes good record-keeping even more important.