What’s the best way to find a dependable software development partner for a long-term project?

I’m planning a new project and need a development team that can handle everything from initial planning to ongoing support. There are tons of companies out there, but it’s hard to tell which ones actually deliver consistent results and communicate well.
For those who’ve worked with external development teams before, what should I pay attention to when choosing a partner? And are there any companies you’d personally recommend based on real experience?