Initially most of the work came through our personal networks. Myself I grew up in Germany and lived in San Francisco for some time. My co-founder had a strong network in Barcelona through his previous job at Deloitte. Today we still get requests through our personal network, however, most of our new incoming work is simply referral based. We don't even have a single sales person. So we try our best possible to do quality work, but also are actively involved with our clients and partners and help them improve their own businesses. This is not just an extra effort on our side, but also makes the whole "work" thing way more exciting and interesting for ourselves, rather than just being a sweatshop that churns out code.
We mostly work with startups, some of them as clients and some as partners. But we also have been recommended and introduced to some big corporations in various industries. And while we don't like the bureaucratic hassle that comes with it, it does give us quite some stability. For example, a big medical company just switched to use as their main supplier for web development, which immediately gave us 30 new long-term projects.
Overall we always try to give a lot, be involved and pro-active and help where we can. In the end you always get something in return. This has been my personal philosophy since ever and always and we've also established this as one of the cornerstones in our company culture.