Everyone has their own way of sending out promotional material to prospective clients. In the past I've had postcards printed and mailed them out or sent out letter size print outs. These days I do things differently. I like something about having more of a hands on involvement with my mailers. I gang my images on letter size doc in InDesign. Then I make a PDF of the pages. Then I go to my local staples to get it printed out in color on premium white paper. I find I just don't get the quality I want off of my own printer. I hand cut the images out. It's not too bad since they are 4up. The images are spray mounted on to cover stock paper that I've used my regular pinter to print my contact info on the back. I feel like the final looks better then the old postcards I sent out. There is hand quality to it and I'm not stuck with way more postcards then I need.
Work changes and evolves and this method allows me more control on the quantity and quality. I make inner envelope by hand from vellum in various colors. I purchased greeting card size envelopes in various colors. It works for my needs. I also will print my business cards on the cover stock to include in my mailer. When I'm doing a mailing of promos I might only have 40-50 prospective clients I'm sending out to. I send out 1x during the summer and 1x during dec or feb. I'm working on my feb mailings now. It's only a list of 10 and growing. I'm particular about who I send out to. I've learned to target and to be cost effective. I can tell by a look at the backlist or current titles if it's worth sending anything out.
The USPS also has a service that you can make single or multiple postcards and save an address book. To keep track of whom I've sent to I made a word doc that has the addresses listed on it for the mailing labels. The file will have their names/address and I just look and see when the file was last modified. I don't bother having my samples returned back normally. I used to in the past but now I just want to save money on postage.
In the future I'm planning on designing a one page promo sheet of various illustrations to send out as a follow up. We'll see how that goes.