Bad communication can ruin everything.
This includes your book signings and other appearances. I'm always baffled by authors who have a Facebook and Twitter accounts, yet completely forget to use them to promote their appearances at bookstores, conferences, and conventions. If you're not going to use social media to create a buzz about where you'll be, you might as well delete your account.
Don't just announce your event on the day it's happening. You should be chatting it up weeks in advance. If you have a blog, write a post announcing it and reminding your readers as the day draws closer. Tweet about it more and more as the days count down. Do the same on Facebook. Send emails to your subscription list. The more you talk about it, the better chance you have of reaching the maximum number of people.
Of course, don't go overboard. You don't want to annoy people, so don't tweet about it every hour on the hour. Just a quick reminder every other day is good enough. Don't be the person who ruins Facebook or Twitter.
Don't announce the event three weeks ahead of time and then never mention it again. This is just as bad as only talking about your signing a day or two before it happens. People have a short attention span. Don't expect them to remember your signing that you mentioned once a month ago.
Make sure the bookstore or conference is doing their part. Most bookstores have their own Twitter and Facebook pages, as well as a website. Check to see if they're promoting your event, too. If not, let them know. Be polite, and just tell them that you just want the event to be as successful as possible.
You must promote your book signings like crazy. It can make a world of difference. Any other ideas for promoting your book signings?