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 🙂