It's not always possible. PR firms are expensive. They also can't promote to a mostly technical audience. That's why you should give talks about your research/development, blog/tweet, post on sites like this. I've found that especially with open source projects, once you get the word out others will be more than happy to be your projects' "technical evangelists" because they love using them.
That being said, if you're a company, can afford a PR firm and you have produced something worth promoting, it usually makes sense to pay others do promote. The Submarine article by Paul Graham really opened my eyes on this: http://www.paulgraham.com/submarine.html (ever wonder where feature stories in mainstream magazines about start-ups, showing happy hackers solving technical puzzles on white boards come from?)
However, I've also seen companies suffer because they've hired sales, marketing and PR staff on "easy" VC/parent company money before they had a working product.
That being said, if you're a company, can afford a PR firm and you have produced something worth promoting, it usually makes sense to pay others do promote. The Submarine article by Paul Graham really opened my eyes on this: http://www.paulgraham.com/submarine.html (ever wonder where feature stories in mainstream magazines about start-ups, showing happy hackers solving technical puzzles on white boards come from?)
However, I've also seen companies suffer because they've hired sales, marketing and PR staff on "easy" VC/parent company money before they had a working product.