I'm seeing more and more gardening and garden center Twitter accounts - nice! Kimmerg sent me a message asking what would make good content for a garden center Twitter feed.
And just today I saw this article - 10 Tips To Be Effective At Marketing On Twitter. There's some good stuff in there, including "you need to know the difference between posting often and spamming," "Make the most out of your account," and "Facilitate conversation!" Great tips.
I'd also say give lots of information gardeners can use. Thinking of the upcoming season, when should I start growing my seedlings? When is it safe to put annuals outside? What's the new annuals you're excited about? Stuff like that.