Pdf Publishing
I am also a part-time pdf publisher. Basically I had a couple things sitting on my computer that I made for my own personal use. Then a friend mentioned, "You know, this could be saleable". So after learning photoshop and every trick there is to know about Microsoft Word (don't like quark), and buying some stock art, I got into the pdf publishing industry.
I was elated that my first product quickly became #1 in its Category (Super-Heroes) on RPGnow.com, and for a while it was the #1 product on Drivethrurpg.com in the "Other Publishers" category. But it did not last forever. For some unknown reason, Rpgnow let Green Ronin release a Mutants and Masterminds product (Gizmo's guide to gadgets) in the Super Heroes section of RPGnow.com instead of the Mutants and Masterminds section.
In the FIRST DAY, that product was #1. Now all I can hope for is to permanently be content with #2 in that category... Until RPGNow lets GR release ANOTHER M&M product in the wrong category! (grumbling finished)
However, I found that some of the other stuff I had (not entire systems, but just short variant rules) for d20 that I was selling for only 1 dollar (my Amazing 1 Page Dungeon and Fatal Flaws) each almost matched my sales for Basic Action Super Heroes this month.
The key, I think to succeeding in this business part-time is to ALWAYS come out with something new. Don't let your name get stale. Also, new products can help your sales of older ones. If you publish non d20 materials, I highly recomend you publish some things for d20, as this is almost essentially free advertising for everything else you make (d20 products get a lot more page views, and the "other products by this publisher" section of the page can lead d20 customers to your non d20 stuff as well.
My d20 1 Page dungeon product got a 5 star rating in its first day of sales. I have decided to produce at least 1 of these every month, and probably at the end of the year sell them in a bundle pack or something.
However, I am not planning on getting rich at this. It is just something to supliment the income I make in my day-job. If things pick up, I would like nothing more than to gear into full time mode, however!