a. find your Tweets so fabulous or
b. find YOU so fabulous
that they want to share your stuff with their own Twitter followers by re-tweeting it. So, because your pal(s) did you this kindness, you might Tweet back a quick, "Thanks for the RT!"—a polite and lovely way to acknowledge someone's support.
After doing this myself for a good while, I saw another way I could show my thanks. First, as soon as I can, I "Favorite" that person's re-tweet, to show that I saw it and am grateful. Next, I find something of my follower's to re-tweet (which takes about the same amount of time as it takes to type out a thank-you message). This is especially helpful to fellow authors, whether Indie or traditionally published, or creators of ANY stripe, as you're spreading the word about their stuff, which is exactly what they need.
What if you can't find something of your friend's to re-tweet or share? You can quickly create something by Tweeting about your friend's Web site, or Sharing a memory of some kooky shenanigans y'all got up to, way back when.
For me, the bottom line is that I want my followers/friends to know that I saw what they did for me and want to return the favor by promoting them as well. I mean, yeah, it's generally more work than I like to do, lazy gal that I am, but it feels good (when I remember to do it!). ;-)
