1) Follow the 80/20 rule. That means you should promote yourself 20% of the time while focusing on other things 80% of the time. Reason being: it's hard to get followers if you only tweet things like, "Hire me!". That's just not interesting, and there's a reason that airlines don't just tweet about their newest and best flights. They tweet about other interesting things as well to grow a following, and so should you.
2) Be helpful. Not everything you do on Twitter has to be related to your business. In fact, most of it won't (see above). If someone asks for a restaurant recommendation in your city, chime in. It's never a bad idea to be a nice and helpful person, online and offline. We oftentimes retweet (=share) things that others ask us to share.
3) Post interesting things. Just posting stuff about yourself is the Twitter equivalent of only talking about yourself on a first date, so don't do that. Share things about organizations and people you like. Most people are aware that retweets aren't necessarily endorsements, but we still recommend reading everything before retweeting it to make sure it isn't offensive.
4) Politically correct? Speaking of offensive: it's almost impossible to never, ever, offend anyone, unless you want to be so politically correct that you are a bit bland and boring. Some linguists prefer to only tweet about business-related topics (which can be controversial enough), while we like to mix personal and private, and yes, sometimes, we use Twitter to briefly complain about bad service from say, our cable provider. We have learned to not censor ourselves too terribly much, but we also don't tweet about overly private things.
5) Have fun. Twitter is the online equivalent of the watercooler, and it's supposed to be fun. Of course, as with the real water cooler, there are people online you'd rather not interact with, and you don't have to. If someone is harassing you, block them. If you don't want to respond, just don't. There will always be people you can't get along with --online and off---and you have to pick your battles. Surround yourself with good, positive people, just like you would in real life.
6) Learn. We can't even tell you how much we have learned from being on Twitter--we follow prominent journalists, writers, activists, politicians, and of course, fellow linguists. It's been an amazing tool, and it's also great for continuing to read in all our languages.