I am in no way an expert on Twitter…. in fact, I am pretty new at it, but one thing I’ve noticed is that it’s a give-and-take kind world. Meaning, you have to do what you expect others to do for you.
Here is an article that I keep referring to quite often. The gist of it is
* listen — retweet
* listen — respond
* promote — but with care.
As far as best practice for the frequency, I’d say increase it as you get closer to the event – like Steve Said – but don’t go crazy 🙂