http://www.usta.com/Improve-Your-Game/Rules/Rules-and-Line-Calls/Foot_Faulting/
From the horse's mouth:
Foot Faulting
Q. Can your opponent call foot fault on you when there is no referee?
A. The Code states that “compliance with the foot fault rule is very much a function of the player’s personal honor system.” If a player is committing flagrant foot faults, then an opponent CAN call him/her on it. But it is a pretty bold move to do so. He/she had better be certain that you have stepped on or over the line prior to contact before making this call.
For the record, habitual foot faulting is as bad as intentionally cheating on line calls. That said, I always urge players to focus on their side of the net, and executing their returns of serve, instead of worrying about whether an opponent has or has not stepped on or over the baseline during the serve.
Q. In a league match (no officials around), what happens when someone is told they are foot faulting, and they (a) disagree, or (b) say "sorry" but do it again. Who gets to decide if they should lose the point?
A. Ahhh… this seems to be a lingering issue…
If they are truly guilty of foot faulting, then they are cheating. If they disagree or continue doing it, then you have a challenging situation on your hands. I would advise you to avoid worrying about their side of the net and concentrate on what you need to do to win the next point.
There will always be players out there who cheat, sometimes- of course- unwittingly. It is your prerogative to handle yourself. If they CHOOSE to cheat, there is not much that you can do to get satisfaction. Maybe try extra hard to beat them.
Q. I would like to know the appropriate course of action to take when an opponent is grossly foot faulting consistently. I found myself in this situation recently, and was not sure how to handle the situation and stay within USTA guidelines.
A. Ask them politely to stop stepping over the line when they serve. You had BETTER be sure that you are in the right though. Remember, you are basically telling them that they are cheating- so be careful with how you handle this accusation.
Taking this a step further, I realize that a “foot fault” is not legal and that it should be considered a missed serve. However, as an opponent, does the foot fault have much effect on your own play? Not really. You might be better served to just concentrate on your side of the court.
Q. When I am serving I tend to foot fault. What should I do so I don't foot fault, but still have a good serve?
A. If/when you are called for foot fault during a match, do NOT change anything. Just simply back up a few inches.
During a match, it is best not to change your technique. Backing up a few inches will not compromise the power or placement of your serves.