You're doing the right thing. You have your business presence loaded up in Facebook and Twitter. You have a solid customer base and some people ready to read your statuses and tweets. Now what the heck should you post?
A recent Mashable article touts the benefits of progressive group deals, exclusive offers, good causes (charity, etc.), allowing input from customers (products, offers, deals, etc.) and rewarding customers who share your statuses/tweets.
If you aren't familiar with progressive group deals, they work like this. If five people "Like" or recommend your post/product, everyone receives (for the sake of argument) a $1 off coupon. If ten people "Like" or recommend your post/product, the discount goes up to a $2 off coupon, etc. That way, there is shared interest and shared benefit, as well as a bit of a scoreboard element to keep people interested in following it.
Exclusive offers and good cause tie-ins are fairly self-explanatory, but how about allowing input from customers? What this means is gathering feedback on how people would like an upcoming product or service to work before releasing it. It can be as basic as asking what type of coupon people want all the way up to asking what new product should be offered. And depending on your business type, that can have a pretty powerful impact.
The final tip regarding rewarding those who help your cause is important. Getting real help and marketing from customers via social media is a big deal. It isn't easy to motivate the masses to include a business plug amidst their socializing. When customers do that for you, make sure you are active enough to realize it and recognize them for their helpful deeds. The more your company operates as a regular human voice on social media, the better, so loosen up and engage the people who contribute to your success.