I've reviewed a number of threads (I've been away a while) and I see several examples of people who tried slow breathing without any luck.
First, no method will work for everyone, even the drugs don't do that! But I've seen slow breathing work for a significant number of people when done correctly and I'd like to put in my two cents in the hope that more people will try it. After all, it costs nothing, is totally safe, and it could be just the thing for you. You'll never know unless you try.
In reading other people's experience with this I almost always see things like "deep, diaphramatic breathing". It's no wonder it didn't work! This description of slow breathing is a misconception that has been spread by people that don't really practice what they preach.
Deep, diaphramatic breathing will do nothing but hyperventilate you and that raises blood pressure! (But doesn't it make sense that something that can raise blood pressure can also lower it?)
The secret to slow breathing is to keep it gentle
. You don't need much oxygen at rest and too much is what hyperventilates. Same thing if you try to force it to the diaphragm.
Another thing to avoid is counting or any other form of inflexible timekeeper. It causes tension and ruins the whole effect There are a couple of commercial slow breathing devices or programs available and they both allow you to set the breathing rate in different ways. But you don't need to buy anything.
All you need to do is breathe slowly and gently in the way that's comfortable for you. Relax first and try to always stay relaxed. Slow, relaxing music can help and it actually helps regulate your breathing. Don't try to force anything and that includes abdominal breathing. When you get into the rhythm of things it will come naturally.
Once you establish a comfortable rhythm, start slowing your breathing gradually. The other thing you need to do is extend your exhale increasingly with the goal of exhaling for twice as long as you inhale. Slow your breathing and increase your exhale bit by bit, just a small amount more every day.
Don't bother trying to count anything. You can sense internally when your exhale is about twice as long as your inhale. And don't bother clocking your rate of breathing. Doing it gently (and not hyperventilating!) you'll surely go below 10 breaths per minute where your blood pressure starts to drop. If not the first time then surely the next, and your rate will keep slowing as you practice.
That's all there is to it, at least to start with. Do this 15 minutes a day as described and you're almost sure to see some effects in short order. If you enjoy it and get results, then you can consider investing in one of the commercial programs and take it further.
When people don't get results it's most often due to bad instructions that make it either uncomfortable or just useless.