Building relationships with candidates can provide the greatest long term benefit.  They can be your future placement, your mole within their current company, source of referrals and down the road your hiring manager. The key to having a good relationship over a long period of time is to build that rapport up front and  nurturing the relationship. It involves someone being the type of recruiter who the candidates can trust.  Some of the recruiter life laws I practice regularly with candidates include:
 
1) return all phone calls and being engaged on the conversation
 
2) be willing to help candidates even though there is not a short term benefit for me. Being uncondition sets up a recipricol relationship.
 
3) treat confidential information  sacred and tell them this will never be repeated and give your word.
 
4)  learn about their personal life, family and friends and remembering details of the conversation for the next time we talk.
 
5) send emails or calling just to 'connect' at least once a year