Partner A: Could I have made that?

Partner B: No, you couldn't.

or "I know you learned to play yesterday, but what time yesterday?" or

Partner A) You know it's tough to figure out.

Partner B) What?

Partner A) Whether your bidding or play was worse on that deal

It's easy to let one of these chestnuts slip out when partner botches a deal, but those sorts of jokes don't change the past and they don't make the future any brighter. One of the worst things you can do as a player is worry about the last deal instead of paying attention to the present one.

If you're playing duplicate, think of it like taking a test with the number of boards you play as the number of questions on the test. You can get a board wrong and still do well just as getting one question wrong on an 18 question test doesn't mean you're going to fail. The problem is if you let that one mistake start a snowball effect.

Here's a couple of uncontroversial statements:

A) Bridge is a game that requires intense concentration

and

B) Bridge is a game where even the best players make mistakes

Those are plain truths. This one is an observation:

C) Bridge players have a tendency to fixate on their mistakes.

But if that sounds like you (it definitely sounds like me), then this one is important to remember:

D) If you are fixating on a mistake, you can't bring intense concentration to the next problem

One of the worst things you can do is, at the end of a deal where, say, partner plays 1NT making 3 is to ask "Should we get there?" or "How do we get there?" That type of question is best saved for after the game otherwise you're looking backward instead of forward. 

A lot of the time we do this to ourselves and need to fight that instinct, but if you bring up your partner's mistakes, you're inviting this terrible mental spiral. The job of a partnership is to get the best out of each other and that means doing whatever needs to be done to bring focus to the next deal. That might mean ignoring a mistake if partner doesn't spot it (Don't say, "The 8 was high!"), encouraging partner after they do spot a mistake (They say, "Ugh! I knew I shouldn't have drawn that last trump." You say, "Let's get them on the next one"), or even just going with partner to get some water/coffee and stretch for a minute. 

Learn what your partner needs to focus. Sometimes partner might need to mentally "dump" the prior deal by going over it briefly, but you should make sure that partner is re-focused by the time the next board hits the table (and try to keep an eye on the clock). 

I'm not saying you shouldn't discuss deals after the fact. Going over boards is a key tool to improving, but trying to do so in the middle of the game is a recipe for disaster. If you and partner are serious, stay after the game and go over the boards (or, better yet, do so over a glass of wine somewhere) and circle any deals that you couldn't work out to ask a mentor, expert, or bridge columnist whose email is listed in the magazine.