\Hi Tim,
I've labored this point before, but I think the best laying tool is the tekobari, which you can get from the Japanese Embroidery Center.
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Of course you have to pick this up and put it down for every stitch, so if that's going to drive you up the wall, get a trolly needle, which will fit onto a finger. Just don't forget and try to scratch your face while you're wearing it. I have one, though I've never used it. I always go back to the tekobari, but that's just me and my love for Japanese embroidery. Actually with Japanese embroidery I usually hold the needle between thumb and third finger, at the same time holding the tekobari with fingers 4 and 5. It saves putting it down all the time, and it's there to grab between thumb and index finger when I need it.
If you find you start off OK, and then the tension goes astray, I'm wondering if you might be letting the thread overtwist. In other words, the thread gets wound tighter and tighter. That will certainly make the canvas show through. The way to avoid this is to drop the needle underneath and let the thread untwist every few stitches, or a better way is to do a quarter turn or so on the needle each time you bring it back up - in the direction of untwisting, needless to say. You'll soon see how much of a twist you need to make with the needle. Keep an eye on the stitches, and you'll see when the twist becomes too tight or too loose.
As another respondent said, it's also possible that you're using too long a thread, so that it simply wears away. About 18 inches or so is probably about as much as you want in the needle, and there are some threads where you'll need even shorter lengths.
As for personal help, I'm in Australia, so I don't know about the Boston area. If your LNS wasn't very helpful, ring around to some others. I'm sure there would be an EGA chapter in your area, or an ANG group, or just some small needlepoint club. Admittedly, needlepoint seems to have gone onto the back burner a bit over the last few years, what with the huge popularity of cross stitch, but I have no doubt there will be lots of needlepointers around.
Another advantage of getting in touch with other needlepointers is that you'll find that there is lots more to needlepoint than tent stitch, and other stitchers will be a real inspiration for you.
John