You don't really use keywords (ie. long tail keywords) as such.
Generally you would scrape the search results for phrases that people would use if they were interested in your offer, eg. if you were promoting an iphone offer you would search for "need an iphone", "want an iphone" etc.
The best way to come up with search phrases is to just brainstorm and think them up. The stemming used in the Twitter SE is quite crude so searching for long tails isn't very effective.
Searching for hashtags related to your niche and scraping the results is another popular method.
