I usually talk to a few people from the audience after the show if I can when I am feeling low about it. I usually schedule 10-15 minutes directly after the show so that I can get vibes from them - actors often like to hear what the audience thinks as well. It certainly keeps me enthusiastic for the show - things like what we can change, who was great etc.
I had a really bum show. Our sole audience was basically cast members families and friends because frankly, we had no budget for promotion, and the subject matter was quite deep and serious. It was hard to keep going with it, because I really believed in the message - it was about AID's and acting responsibly to stem it's spread. I had a very close friend recently diagnosed after she was raped and was doing the show for jack all money (during a tight time in my lifet), and I felt that this message that was close to my heart should be shouted from the rooftops, and it was so discouraging that hardly anyone showed up. In the end I approached management and asked for 10 tickets at a reduced price and gave them to random people on the street on the condition that they came to the show and talked to me afterwards. When they appreciated it, I got the strength to keep doing the show. Obviously, I would only do that on a show that I was really passionate about - mainstream shows don't fall into that category.
Just listen to the audience. Sometimes it backfires and you hear "Well that was the biggest load of crap I have ever seen", but usually you hear some nice things.