If you have a following on twitter right now and are trying to promote a product, site or service there are several ways I have come across. Some are ok, some horrible and some work well.
1)Repeat your tweet in the status area very frequent but you will have to alter it slightly to avoid twitters spam filter. I have heard many internet marketers saying that tweeting the same post 3 times is enough to increase CTR(clickthroughrate) but not enough to annoy your followers that only have a few followers and will see all 3 tweets in their window.
2) Use your affiliate link and product description right in the status window. This rarely works and has horrible CTR. I have used this successfully but maybe once a month and only if the product is sold through a well known and trusted e-commerce store(like amazon) and its a killer deal like 50% off. If you try to promote sales pages this way people will likely unfollow you if all you do is bombard them with sales page offers in your tweets.
3)Offer freebies and link to an opt-in page. This works well as long as you have a targeted follow list you have a killer freebie. I use this frequently and have gotten conversion rates from my total list of around 3% which is pretty good for twiiter.
4)Promote reviews and articles and link to blog then sell them there. This works well also since you are providing value upfront and does come off as being pushy.
5)Promote your Facebook account or other social media account. This works well also. A tagline that works well is "Lets expand our conversation on facebook" or "checkout the pictures of____". Once they are on your facebook account you can expose them to your offers.