Go to conferences where you can meet either of these two groups of people: potential clients and potential referral partners.
Potential partners include people who serve your potential clients, or have relationships with potential clients, and aren't competitive. For example, someone who does PR for startups might benefit from connecting with someone who does accounting for startups.
Ask people you respect for their suggestions for which events draw the most interesting people and might be the most effective use of your time.
Once you find a trade show that may have either of those two groups of people, do some research to determine who will actually be there. Check to see if a guest list is publicly available, or if a list of speakers or booths is available. Search Twitter by the event's hashtag to see who is talking about the event and what they're talking about.
Before you arrive at the tradeshow, set up meetings with people who you know are going to be there ahead of time. This will help you make the most of your time and ensure you meet face-to-face with the people who are most important to you. Do some research on the people on the guest list that you want to meet including what they look like, so when you see them you can approach them.
When you meet someone new, ask them if they know anyone else at the event that you should talk to, or if they've met anyone interesting so far. Try to connect people at the conference to each other. If you meet an accountant who works with startups, introduce them to a lawyer who works with startups, or to a startup.
Get contact information from everyone you meet so you can follow up with them after the event.
Related: SXSW: Don’t Let Contacts Fade