This question really does depend on if you're talking about B2B (selling the technology to other travel companies) or B2C (you have a compelling travel site or app that a customer can directly use).

On the B2B side, nothing beats patience and focussing on making an amazing product that people understand why they should use it (the value proposition), keep using it (retention), and then recommend and tell others about you. In the travel space, word travels fast and you'll build up a reputation for having something valuable.

To accelerate that you'll want to establish yourself as a company that can add value in the space through speaking at industry events and conferences, hosting your own events where you educate and inform on a particular topic and capture contact details / briefly chat to the attendees, and publishing regular, quality content that again is interesting and engaging.

I'd invested in numerous SaaS based companies of this nature and this approach is by far the most successful, albeit often a slower burn than direct marketing.

You should also look at using Hubspot (www.hubspot.com), and reading the blogs of Tom Tunguz (http://tomtunguz.com/) and Christoph Janz (http://christophjanz.blogspot.co.uk/) that focus on this in particular.

For B2C, it's a very competitive space and happy to go into that too - depending on what your focus is..