Gretchen Rubin has a list of 8 clues that you are being boring. She says her list is "utterly unscientific" but I think she's right on. Most of them are obvious, but boring people seem unaware that they are boring.
She also has a list of topics to avoid if you don't want to be a bore:
1. A dream.
2. The recent changes in your child's nap schedule.
3. The route you took to get here.
4. An excellent meal you once had at a restaurant.
5. The latest additions to your wine cellar.
6. An account your last golf game.
7. The plot of a movie, play, or movie–in particular, the funny parts.
Of course, these topics are Gretchen's pet peeves. I just happen to agree with her about all of them (especially dream and golf). Some of these topics, however, might not be boring if you tell them to the right person. Number five would be interesting to a fellow wine collector, for instance.
UPDATE: Please participate in this one-question survey so we can find out how many people get bored when other people describe their dreams to them. (The dumb survey app I'm using will not show results to everyone. As of 11:24am PST, 362 people have taken the survey. 60% people have answered Yes, and 40% have answered No. I will update again later.)