For a realtor, I guess you'd have to first target those prospects that are more immediate... so someone you know is going to be buying or selling in the near future. Then find out something about them, and what's important to them. It doesn't have to be an expensive gift at all. For example, I had a prospect who I'd met with at his home because he'd had surgery on his foot prior to our scheduled meeting. While I was there, I met his very affectionate Boston Terrier, and we got into an extensive conversation about how he's had Boston Terriers since he was a boy. About a month after our meeting, I came across an inexpensive miniature statue of a Boston Terrier. I picked it up in the store and mailed it to him, with a note that I knew he'd appreciate it.
The important thing is that the gift is sincere. If you take the "gift" step in a superficial way, then it seems phony, and the idea will backfire.
More specifically in your line of work, Realtor... I recently moved. I know several real estate brokers in the area. Some time before I was ready to list my house, I'd gotten into a conversation with one of the realtors about wine, and we realized we both enjoyed the same types of red wine. He dropped by one day with a bottle of his favorite red wine and said he was anxious to know what I thought about it, once I had a chance to taste it. It was a sincere gesture that stayed with me, and when I was ready to list my house, I did list it through him.