The first thing is to be SURE before challenging a call. If your ball was clipping the back of the baseline and got called out -- don't challenge it. It needs to be obviously in. Also, be somewhat forgiving in night matches. The shadows of the ball caused by the lights can be very deceiving to both players when making line calls.
Anyways... the first time I think a call was horrible, I'll ask... "Wow, really? How far was that out?" I'm really looking to see what the response is from the opponent. Looking for body language. Trying to see if it was intentional. If it happens again, I'll ask the same question again. If it happens again, I'll tell the other side that I really think their calls are bad and that I'm thinking of resigning the match.
But honestly... I see way too many players getting upset about OUT calls that they think clip the back of the baseline. It is ridiculous. If you are that concerned about it, then work on your game... get the ball safely inside the baseline.