Referrals are vital. If you do a good job then candidates will tell their friends and colleagues about you. You could think about offering incentives to people already on your books so that if they recommend you and you place their friend in a position they get a reward or gift. Everyone loves a freebie!
It's important to remember that you are a recruitment professional, and therefore a salesperson. Even when you are out of the office you should be constantly on the look out for leads. When you are introduced to someone new it shouldn't be too hard to find out (without it being too obvious!) if they are looking for a career move. Make sure that friends and family know your role and are ready to mention you to anyone that might be looking for work. Even if you can't help them, you might know a consultant that can, and in return they might return the favour in the future.
Don't underestimate the power of networking. Be on good terms with other consultants both in your company and others because you never know where the next lead might come from. If you approach a potentially good candidate and attempt to sell them a position but they're not interested, try and get a referral from them for someone they know that would be perfect for the role.
One of your main aims as a consultant should be to have a large network and large pool of candidates so that you can quickly and effectively find the right person for the right position, increasing your effectiveness as well as how you are perceived by both the contractor and client. Don't wait until you have a vacant position before you try to build up your candidate database!