I have made a number of projects with dovetails and am chagrined to rate myself as only OK. The major problem is consistency. If I cut 4 joints, usually one will be great, one will be very good, one will be good and one will be poor. I am not improving. There are three ways to go. Buy a router jig? No. Use my bandsaw? It may come to that. Improve my hand tool skills? That's my preferred choice if I can do it.
The first step is identifying the root cause of my problem. In my case, and I suspect for most woodworkers, the root cause is not sawing to the line. To make a great dovetail, you have to be able to saw very accurately to a line that is perpendicular in one dimension and angled in the other. Bob Rozaieski says that paring to clean up a dovetail cut doesn't work for him and that has been my experience as well. When you think about it, the fact that we can learn to control a handsaw to make a cut that is within a few thousandths of a line over a distance of 3/4" is amazing. It's not surprising that it takes an effort to do it consistently. I keep reminding myself of the time it takes to acquire a physical skill--a golf swing, kicking a soccer ball, playing a musical instrument . . . As I was researching this subject, I read somewhere that James Krenov wouldn't let his students make a dovetail until they had sawed to the line 1,000 times. I don't know about 1,000 but this is the direction I am going to go. This is almost the opposite of the "five minute dovetail" approach where you just practice making a single dovetail quickly.
The mechanics have to be right. The most important thing is starting the saw just right. If you do that and relax, the cut will come out just fine. Just right has two pieces: side to side and up and down. I found my issue. I am left handed and I find that I consistently start the saw just right side to side when I am sawing on the right side of the line because then I can look over the top of the saw and see the line right next to the teeth. The problem comes when I try to saw on the left side of the line. I have a big thumb and it obscures the line. The changed relationship of my eyes to the saw may be a factor as well. It has occurred to me that a solution is to mark the dovetails on both sides of the board and turn it around for half the cuts, but-- that would be wrong. :) So this is my challenge.
I didn't find anything online or in my books that is very helpful except a hint. The hint was to start the saw at a slight angle and then bring it down across the board. That was it. The saw was sometimes skating a little and I couldn't see it. I find that if I use my thumb to start the cut on the front edge of the board and then immediately remove it as I bring the saw down in a continuous stroke the problem appears to be solved. Since I can't walk and chew gum at the same time, it's helpful to concentrate on the line across the top of the board for a first light stroke and then concentrate on the line on the face of the board thereafter. I think if I make practice cuts 1,000 times I'll have it. It will be one fluid motion that I don't have to think about. That's what mastery is.
What about up and down? If you cut pins first as I do, it doesn't really matter if you go off the line and, in fact, you could even argue that some inconsistency gives a hand made look. I'd like to saw to the line, however. Maybe it's me, but I think it's easier to saw straight up and down. I just angle the piece so that the cut is perpendicular to the vise jaw. It takes only a few seconds and is easier for now. I use a Lie Nielsen dovetail saw with .003" of set, so you do have the ability to correct slightly at the very first; after that it's bad to even try. The experts are right that a very loose grip on the saw is critical, so loose that it would drop out of your hand if it weren't in the cut. The pins, which have to be accurate in both dimensions if cut second, are easier for me---that's why I do them second. For me, the whole debate on pins first or tails first comes down to this---make the cut you are better at second because that's where accuracy is most important.
This is likely obvious to most of you. I think the larger point is that if you want to get really good you have to learn from your mistakes and put in the time and effort to master the details of the skill. You have to practice a lot. I try never to make a mistake without learning from it. The fact that it takes considerable effort to learn to use hand tools well is part of the fun.