Yes, the gold standard is employee referrals. Pretty much every company we've spoken to has told us they're the best source of people, but they don't get enough of them.

Part of what makes them so good is that the person making the recommendation (i.e., the employee) knows both parties well. I think we have a good start to solving half this problem, since we're developing real relationships with the companies and being selective about whom we work with. The other half -- actually getting to know the people applying -- is probably the biggest challenge we face (next to meeting/finding people in the first place).