It's one of those things you'll get to deal with on a daily basis. And after a while it becomes like water of the proverbial duck's back and as has been said before a little empathy goes a long way.
You'll find people complain most when their expectations aren't met and as products are changed and downsized quite frequently at BA it happened quite a lot. Once they have got used to the new catering and accepted it most will stop complaining. Of course you'll always get those who complain for the sake of complaining but I think most will agree these people are beyond help.
Believe me being crew is a great confidence builder, you don't have any choice but to deal with situations. It does take some time but you'll be surprised at how you can deal with almost anything. And most importantly learn from your colleagues and how they do things, but you should always ask yourself whether you would handle the situation in the same way? Not all cabin crew are great at the old customer service thing!
Good Luck
tsb