Weirmonken: Thanks for the extensive list! I'm sure that represents an awful lot of time and effort on your part, and I certainly appreciate it.
To answer your specific questions:
How did you get involved in puzzle design?
I've enjoyed puzzles for as long as I can remember. As a little kid I liked doing mazes and find-a-word puzzle books, got into crossword puzzles when I was a bit older, and currently enjoy sudoku puzzles. I remember freaking myself out when I was about 8 years old and on a cross-country camping trip with my family, looking at the name "PONTIAC" on the car driving in front of us, and realizing it was an anagram (although I wasn't aware of that word at the time) of "CAPTION."
As for puzzle design, the original "Challenge of Champions" adventure was my first real stab at puzzle design for an RPG. The core concept is something I took from "Project X," a leadership challenge that they send you through as an Air Force Captain when you attend Squadron Officers School. It's a series of challenges, usually of the "get from point A to point B in so many minutes using only the equipment provided" sort, and you get graded not only on your ability to meet the goal but also on how you work with the other members of your team, make use of the provided equipment, etc. After going through it myself (which was definitely my favorite part of SOS), I wondered if such a concept could be carried over into AD&D.
And I'll let you in on a little secret: although I had been playing AD&D for years at that time, I was fairly uncertain just how one went about writing an adventure that was specifically geared for "X level." (Remember, this was before the concept of Challenge Ratings.) One of the reasons the "Challenge of Champions" series was designed to be for characters of any level was because I wasn't sure just what level it was appropriate for otherwise.
Do you have any suggestions for DMs who want to design their own puzzle elements into a scenario?
The best advice I can give to a budding puzzle-writer is to be familiar with the types of puzzles that might be appropriate. For me, with the "Challenge of Champions" series, the first thing I do is decide whether I want a unifying theme. Usually, this is the final scenario, which often hinges on the names of the previous scenarios in the challenge (and then using a word-based puzzle for the last scenario). In such a case, I need to design the last scenario first, so that I can then figure out a way to create the other puzzles with names that are useful for the solving of the last scenario.
Another useful way to generate puzzles for RPGs is to pore through the magic items and spell descriptions and see if there's a "hook" that can be used to generate a puzzle. Often, this is using the spell or item in a way it's not usually intended, or focusing on a lesser aspect of the spell or item. (For example, rope trick is best known for its extradimensional space, but it's also a perfectly good way to create a vertical anchored rope for climbing.) Don't forget the cursed items, too, as some scenarios involve trying to figure out which of two identical-looking items is the "good" one.
Are there any books you found particularly helpful in this regard?
Not off the top of my head. I remember there was a good article in Dragon some years back by Mike Selinker that had some very useful advice for puzzle design in dungeons, though.
Do you have any further plans to write material for RPGs?
It's funny in a way, but when I was at the peak of my Dragon and Dungeon freelancing I was between campaigns and didn't have a gaming group for the most part. Of course, this gave me plenty of time for writing articles and adventures for publication. With the advent of 4E and my unwillingness to move on from 3.5, my freelancing came to an abrupt halt, but at around the same time I started up my present campaign, and the extra time I've had to write adventures for my home group is something I wouldn't have had if I were still freelancing at the same rate I had been. So it's kind of balanced. Of course, the adventures I write now are very focused on my particular gaming group, and aren't fit for publication in their current form. (I wrote a really cool tomb delve, for example, but it had a puzzle door that involved the names of various deities in the Greyhawk pantheon, so that's out.) I had pretty much figured my freelancing days were over, but with your list above, who knows? :) Thanks again!