Glad you guys like 'em! :grin: I have a few tips, I suppose. Let's see if I can give them in point form.
1. Start with as good a reference photo as possible. Tirerack.com has a great selection of tires with dead-on shots of most at 250x250 pixels. This is what I use for most of mine.
2. Take a good look at the tread pattern. Try to find the limits of the pattern. Many patterns are quite small, and can be tiled to get the desired tread effect.
3. Work in grayscale. I use this on all my treads and it simplifies getting the pattern to tile correctly. Also, lowering brightness/contrast is useful in limiting the change in lighting across the tread.
4. Make sure your pattern tiles properly! I may have posted one tread that doesn't tile properly, I'm not sure. Now, I always make it a point to test the texture before I 'finish' it.
5. Use gradients on a separate layer to make lighting even more uniform. Use the layer's transparency to get the desired effect.
Hmm... Those are all useful, but now it seems a bit disjointed.

If there's enough interest (like one person or so...), I could do up a tutorial of how I do a tread from start to finish. Let me know if this would be useful. In the mean time, how about some more treads? :grin: