Answering questions from people about the way your industry or business functions is a sure-fire way of making yourself look like an expert (provided you actually know the answers). Look through topics related to your business on sites like Quora and LinkedIn and set up a saved search on Twitter and look at hashtags. Join industry forums and participate in discussions. Use web search and Google alerts to find more questions on those topics. Don't just answer for the sake of getting your name in: you need to actually add something informative to the discussion.
Especially on LinkedIn and Facebook, join groups related to your industry and again, engage in discussions. Connect with influential members of those groups and learn from them.
Share interesting articles about your industry on Twitter (as often as you read them), Facebook (more judiciously, unless you want to be hidden or un-friended) and LinkedIn (also judiciously). I especially recommend this for Twitter. Bonus points if you find somewhat obscure sites to link to instead of the leading industry blogs that everyone reads anyway.
Comment on News
Even if you have a limit of 140 characters, you can make some observations on big industry news. A hashtag is trending due to an event? Jump into the conversation. Bonus points if you can be informative and witty at the same time, and minus points if you are obnoxious.
Mention What You Are Working On
In your status updates, talk about your work. I might, for example, say something like this:
Often, people in your industry might reply and there, you have a conversation. If you make your tweets smarter than mine, others will follow you in the hopes of learning from you.
Share Your Blog Posts
Yes, I had to go there! If you blog, definitely use social media to promote your blog posts and take the conversation to those sites as well. Tweets and Facebook comments have to an extent taken the place of blog comments, so take the conversation to your audience. You can also use intriguing quotes from your blog posts to tweet, with a link to the post, of course.
Whatever site you're on, make sure you're responding to any comments and carrying the conversation forward, and not merely broadcasting your views. The most respected social media influencers are those who talk to their followers and use feedback to refine their own views.
Now get going!