What do you look for first when you start a new puzzle? My personal favorite is to examine each row of 3x3 boxes in turn, and then each column of 3x3 boxes, always looking for numbers that appear twice but not three times. It is often easy to solve several cells this way. And repeating the process, even several more times, can solve more and more cells. Is this a common technique? I haven't been doing this long enough to know.
@TallMike, I usually start by going through all the numbers sequentially (look for any obvious ones to fill in, then obvious twos, etc.); if that's productive I run through them again until I feel I've exhausted them.
Jim, I used "your" method for a while but realized that, at least for me, it is quicker to first find a row or a column of 3x3 boxes with only two ones, or two twos, or two threes, etc. and then see whether the position of the third instance of that number is obvious without looking at any of the other numbers. If you try it, I think you will see that it goes really fast. Repeating this method to exhaustion usually leaves only a few more obvious numbers to be found by your method. Too bad we can't have a race sometime!
Jim, I think the difference between our methods becomes more apparent if you consider one of those expert puzzles where there are so few numbers provided that it is hard (for me) to believe there is at least one obvious number to be filled in. Maybe you never get that feeling, I don't know, but I experience a need for immediate reassurance, and once I have found that first quite-easy-to-find number I feel a lot better.
DING. I caught myself with the wrong idea of what numbers were needed in a row. So I backed out several moves in the area, at least one of which was mistaken. And got finished without a mistake. Because of the quantity of loners out of four, I'll give it a 20.