A couple of hours before the party, though, my friend emailed me and said that she was exhausted, so it might take a little longer for her to rally to go to the party -- was I ok with going later? I put myself in her shoes and thought about how awful it feels to go out when I don't feel like it and I sent her an email back saying that I would be ok going alone if she would rather not go.
She was surprised that I would make the offer -- I don't love going to parties alone -- and she triple checked to make sure I was ok with it. I told her my policy: I don't make offers that I wouldn't be happy (or at least ok) with because I want people to believe that I'm ok with the offers that I do make.
A similar scenario presented itself a few days ago. A friend and I were planning on having dinner, and I was really looking forward to seeing him again after a long absence. Shortly before dinner, I got an email saying that he had had a long, tiring day and would likely be low-energy when we got together. I told him my policy, and said that I wasn't going to offer to reschedule -- why invite disappointment? -- but that if he wanted to make that offer, I wouldn't offer a lot of resistance. He clarified his comment by saying that he didn't want to reschedule, but was just giving me the heads up that he wouldn't be overly energetic. (His phrase? "I won't be juggling sparklers.") Disappointment averted!
I developed this policy after years of making offers I felt compelled or obligated to make -- offering to forego the party I was already dressed and ready for, or offering to go to a noisy, overcrowded bar when I really just wanted a quiet dinner. I did it because it felt like it was the "right thing to do," but it always left me feeling crappy afterwards. Sure, there are going to be times when things are going to be cancelled or plans will fall through -- that's unavoidable. But I don't need to go out of my way to invite people to disappoint me, especially (as happened in the second case) it wasn't on my friend's radar.
I also do this because it can send mixed signals to the other party. If I offer to cancel or reschedule or change the plans in a way that I don't want to might make the other person think it's what I DO want, and they may accommodate me accordingly.
So I just stick with honesty. And I make sure I'm consistent and that my friends know about it. So far, it has worked really well.