Twitter is a social network where there rants and rambles of almost everything,promoting yourself,promoting your product and for friends,but its not so wise to promote your product directly as people don't like that by trying to misdirect them,so how do you promote your product using twitter?Basics should be considered and main factors.
Indirect Product Promotion-be genuine,don't go overboard and misguide your visitors,people need assurance,or likely you might loose them.
Topic-when you are trying to promote a product at least let it be like you are answering a related comment or question or talk you are having ,everyone hates spammers.
Balancing-try and create a conversation,don't focus too much on what you are selling,nobody wants to be redirected to a blog or website that is just for promotions,and if you promoting a particular product let the information you have be useful.
Trust-you need to gain trust from your visitors,let feel the need of wanting to know more when they visit your site,once they build that know that word goes round,and once you get a regular visitors let them stay,and try to answer their queries or doubts as genuine as you can.
Sometimes you can offer to direct them to other blogs that you think are genuinely good for more information,this way you create a good relationship with your guests and they trust you to give them advise.