If you have trouble condensing what you want to say to standard 140-character tweets, you can use TwitLonger which will post the first 100 or so characters along with a shortened URL to the TwitLonger web app.
There are many apps that use the TwitLonger API; I just tried Tweet Longer for Android which works well enough, but there isn’t much polish or features.
It can be useful, but kind of skirts the purpose of Twitter to begin with. Basically, it’s TwitPic with text rather than photos. I don’t think I’d regularly use it myself, but it might be helpful for some of you.