1) Dont be loud. Same social rules apply online as offline.
2) Only follow people who tweet content you care about, following lots of people for no reason is like inviting strangers to your birthday party.
3) Sometimes it is best to just listen and watch then it is to tweet and blab about nothing.
4) It is a marathon, not a sprint, I have been using Twitter for 2 years now and this is something I always keep in mind.
5) Take time to interact and answer @replies, I do this with every one I get unless they are from a spam bot.
6) Do not automate your updates UNLESS you are a news website or offer niche information that people will find interesting.
7) Keep in mind that 50% or more of your followers are spam bots or similar and do not EVER look at your tweets.
8 ) Using #hashtags is important to add your thoughts to an ongoing conversation (event/conference/city) but do not over use them. Google ranks Tweets without #hashtags as having more authority.
9) Your Twitter username is used as your title tags, so either use your government name, business name, or name of a product/service you are trying to rank for. Smaller usernames are better as they take up less spaces in the 140 character allotment of tweets.
10) Your Twitter bio is used as the meta description so fill it with keywords related to your business or blog.
11) Include your email address in your twitter bio to allow for people to easily contact you for business.
12) Use a friendly and easy to understand profile picture, I have a cat in mine and Im smiling, many people comment with positive feedback on it.
13) Instead of retweeting everything you find interesting (as that gets noisy to your followers) try using the favorite button and reading it over again later. At which time send out the retweet with a comment of yours.
14) If you really want someone to follow you dont ask them to follow you, give them a compliment and interact around topics you feel they will like.
15) Have fun and dont take things very seriously, sometimes people comment negatively or in ways you do not want to hear, just think to yourself water off a ducks back ie it doesnt matter.
16) Whenever possible use full URLs on content sharing vs. shortened ones, as this will give you better SEO results on the content and increase click-throughs.
17) Dive into conversations related to your business or Blog using Advanced Twitter Search to find the best ones to join. Dont broadcast to them, but rather answer questions and be helpful.
18) Use a second computer screen and a system like Tweet Tabs to follow many conversations on the go.
19) Be conscious of your API limit on the Twitter application you are using and make sure you do not run out if you are doing some important customer service. If you do run out you can always use the web version.
20) Dont be afraid to ask people for help.
*Advanced user tip:
Google ranks Tweets by processing the number of followers you have vs. following. The more followers you have the more weight is given to your Tweets in the SERPs. The more followers each one of your followers has increases this further. Therefore if you want to get highly technical with your Tweeting, only follow people with more followers then following and get them to follow back. Connect with influencers. Each person you follow is an outbound link and each person following you is an inbound link, link authority applies here also. Obviously this is an important factor to consider, but being social is the name of the game.