I just quietly ask that they check the order for me because I believe there is a mistake.
I don't do any shouting. Getting the server upset is not going to do any good., I have seen customers shrieking at waiting staff, all that makes me want to do is step up for the server, and call the customer an idiot. Having a quiet word has given me free items. One place gave me a complete refund and a free meal voucher as well as cinema tickets for my wife and I.
A few times I have noticed waiting staff miss off a drink charge, I always point out ommissions to my favor too. Quite often managers and staff have thanked me and waived the charge anyway. That is nice, but I also feel much better when order disputes are amicable and friendly.