1. If you have to break a thought into two tweets, end the first one with […] and begin the second one with […].
2. If you use an old-style retweet for something and want to add your own commentary to it, add your commentary before the RT text. For example: “I really like this movie, too! RT @weaselpunk I just watched Mr. and Mrs. Smith for the kabillionth time. Swoon!”
(I think that you should use old-style retweets precisely for this reason—it allows you to add context and commentary for your audience.)
The text “RT” creates a nice wall between your text and the person you are quoting; it makes it easy to tell who said what. When you recommend something to someone, you typically don’t say “Watch this first and then I’ll tell you why I like it”—you tell someone the reason they should watch it first.
Don’t RT someone to reply to them. There’s a reply button for that. Don’t quote someone when replying to them.
3. The more you deviate from proper spelling and grammar, the harder your tweets are to read. Some comments just can’t be “twitter-sized”—so use one of the services that allows you to post a longer comment and link to it, break your comment up into multiple tweets, or email/IM/something else the person you’re trying to talk to. If the majority of your tweets are jam-packed with shortenings (shrtngs, u c wat i mean?), it doesn’t matter how jam-packed they are with info and commentary: they will get looked over. A “u” and some digits once in awhile aren’t a big deal, though!