I know you are not having fun, but would you please wait to complain?

[Texas Mom]

Our solution to constant complainers was not to take them anywhere.

^
My (deceased) father did this one too many times, in spite of bean dipping and talking about it beforehand.  My DD got tired of being embarassed and finally told me to quit inviting him to events.  I did - he missed out on a lot of things in his only grandchild's life because he refused to keep his mouth shut until he got home. 

By the way, both were older women. I hope this isn't a trend! 

 
Age isn't the issue - I know people who behave in this manner and they range in age from 25 - 90.

[Luci45]

Our solution to constant complainers was not to take them anywhere.

^
My (deceased) father did this one too many times, in spite of bean dipping and talking about it beforehand.  My DD got tired of being embarassed and finally told me to quit inviting him to events.  I did - he missed out on a lot of things in his only grandchild's life because he refused to keep his mouth shut until he got home.

Luci says: The culprets were my step-mother and someone else very loved. They were warned. Too bad they missed out, but if the youngster was happier, that's the way it goes. (PS We are the grandparents now and know our options. Our problem.) 

By the way, both were older women. I hope this isn't a trend! 

 
Age isn't the issue - I know people who behave in this manner and they range in age from 25 - 90.

Luci says: Whew! Thanks for the comfort!

[CakeEater]

My grandmother has always been a complainer, but now that my grandfather has passed away, we all get to hear a lot more. I guess he mostly bore the brunt of it before.

It really makes people visit her less often. Sitting through an hour of constant complaints isn't the way most people want to spend an hour.

OP, I'd start counting the complaints out loud as PP suggested. Maybe keep count of the ratio of good to bad. 'That's 1:8 now Mom.'

[Auntie Mame]

I have a sister?

The worst was when we were at "Famous Musical that I Love and Had Never Seen Live".  She bought me the tickets as a gift.  Then complained the entire time.  "I thought our seats would be better.  The set isn't very elaborate.  That guy in front of us is so tall" And on and on and on and on until I wanted to scream and throw her off the Mezzanine.

In stead I turned to her and said "Mom, I really appreciate this generous gift you got me and I am having a really great time".   With that phrasing she couldn't turn it into a personal attack, she couldn't argue because i was talking about my feelings and shut her down.

[doodlemor]

I had a close relative who tended to do this, too, OP.  It certainly takes the sparkle off of things, doesn't it?

I like the PPs' idea of tallying the number of complaints, even if it is just a mental tally.  I also think that the complaining would be an occasion for "So you say.'

After the first or second complaint, I would reply with "So you say" to each and every complaint.    As this wore on the reply would become more like "So.     You.     Say."  At some point mom is likely to notice your repetition and call you on it.  At that point you can say, "Mom, do you realize that you have complained xx number of times since....we got here, x o'clock, whatever?" 

You could then ask her if she wanted to leave, do another activity, and so forth.  If she is a reasonable person at all it seems like calling this to her attention would make her want to get out of such a bad habit.

[peach2play]

She likes to complain.  It's not that she isn't enjoying herself, but for so long complaining has gotten her sympathy and rewarded.  Stop rewarding her.  When she complains, turn around and walk the other direction.  When she asks why tell her the truth, that you want to enjoy the day and listening to her tear everything down is not enjoyable.  Then, don't take her the next time you do a similar activity.  She'll either learn or she won't but you'll be better off for it.

[rose red]

She likes to complain.  It's not that she isn't enjoying herself, but for so long complaining has gotten her sympathy and rewarded.  Stop rewarding her.  When she complains, turn around and walk the other direction.  When she asks why tell her the truth, that you want to enjoy the day and listening to her tear everything down is not enjoyable.  Then, don't take her the next time you do a similar activity.  She'll either learn or she won't but you'll be better off for it.

I agree with the bold.  I used to feel bad the complainers are having a bad time until I realised they are actually having a good old time.