Know the film you are writing about backward and forward as time permits. If you do not have access to the film itself, immerse yourself in the marketing materials to learn as much about the tone, plot and theme of the movie as you can. The more these things are embedded in your subconscious, the more true to the film your taglines will be. You can also check out "imdb.com" for up-to-date information about movies, including those in pre-production.
Make lists of words associated with the title. Go to "dictionary.com" for various definitions, further synonyms and word ideas. Visit "clichesite.com" to see if you can incorporate a witty cliche into your tagline.
Glean inspiration from successful movies. Chances are a movie that was a huge success owes its success in part to a great tagline. Notice any commonalities in the structure of these taglines.
Aim for succinct and memorable. Producers want their film, of course, to be memorable, so if your tagline is memorable, they will be interested. If your tagline is memorable, but too long, they will not be interested. The key is packing a lot of "razzle dazzle" into as few words as possible.
Try to end up with at least 50 decent taglines per film. You should create many more than 50, but should throw out a large portion of those. Producers will want many options, and even if you are not submitting your taglines yet, writing at least 50 will be a good habit to develop for future submissions.