I am thoroughly convinced that I have landed my biggest job to date and kept my biggest clients by writing thank you notes. It shows that you care. It shows that you think a little bit more than other creatives in your field, and have a human side. People like this. I was asked to present a portfolio to a new client recently, and thanked them for their time with a hand-written note afterwards. They had the budget to fly someone in from Europe if they wanted to, and I know that they've worked with other photographers who's work I admire in the past. They certainly didn't pick me on price, I can assure you of that. I was not the only option on their table. Yet, for some reason I am completely convinced that my simple hand-written thank you note was a contributing factor in me landing that job. It costs next to nothing - I try to take a small percentage of my income and give it back to my clients - don't just mail bomb them with advertisements and phone calls asking them to use your service. Put a little time and effort in, whether it's either a thank-you note or a small gift. Doesn't need to be big. Just has to be human. I guarantee you'll see results.