Take your referral partners (and potential referral partners) out to lunch Or breakfast, or coffee. This is my primary marketing tool to build and maintain relationships. My goal is two a week. When things are crazy busy, it may only be once a week but I try to make it up when things are a little slower by doing more.
Eating together gives you time to really get to know each other and find out ways you can help their business. I will also ask a Realtor I know well if there is someone else in their office they would like to invite. This has been a great way for me to meet new Realtors.