Some random thoughts (not entirely my own, as they've been bounced off a few others over the years) about giving readings. These come from one that attends many readings and has, over the years, determined what I think works well and what maybe doesn't so well. Take them how you will.
1. Read for at least 10% less time than you think would be appropriate. Yes, at LEAST 10%. Those attending might be there as friends, might be there as potential buyers of a book they heard good things about, might be there as patrons of a great store or readings series--they most likely are not there because they really wanted to be read to for 20 minutes.
2. Be loud and clear. While I've rarely been overly happy at the end of a longish reading, I've never been happy at the end of one that I had to strain to hear the reader.
3. Find a place in your work, be it a story, a novel, or non-fiction, that demands your listener NEED to find out what happens next. Do NOT tell them what follows--make sure they are left on the edge of their seat so they rush to the table to grab a copy of your book. To me, this probably could easily be number one on the list--there is rarely something as disappointing at a reading than to have a reader his a note that has me think to myself--Perfect ending--only to have their voice continue on with the next sentence.
4. If you write humorous material, TRUST IT. Leave some room for the listeners to laugh before you move on after your punch lines.
5. Q&A's are great. If you have 30 minutes slotted for your event, read for between 10 and 15 minutes and use the rest for allowing the audience to ask questions--it brings a more personal experience for those in attendance--to me this is similar to being on a panel--those in attendance usually have something they want to know, and it rarely seems to be what you've prepared. If those watching you read get more involved via a Q&A, they are probably more likely to pick up your book.
6. Always remember to thank everybody for coming out and the store/series for having you.
Again, these are just some thoughts, not from one that does readings, but from one that has attended many and founds trends in those that I remember fondly.