What I mean when I say a fairly large pipe-bowl is like a normal (old man's) tobacco pipe. This is large compared to what most people use for marihuana smoking these days, but in practice, I am only talking about a bowl which is say 2cm diameter and say 2cm deep.
Crushing the dried leaf and packing it into the bowl, anywhere between two thirds and completely filling the pipe will be enough leaf for smoking.
Obviously it pays to take as many steps as possible to ensure a good effect, but even so, I still find it an extraordinarily variable thing. I think a lot of people do. I still get occasions where nothing much seems to happen. With that said, here is some further detail about my experience of using a water pipe.
One of the things I had to particularly get used to was taking deep inhalations. I find the smoke rather hot. Still, with practice, usually by breathing in real slow and deep, taking a tiny breath of clear air through the side of my mouth if it gets too hot, I have found that the strongest effects come if you can do it in a few big inhalations. A lot of the advice that you'll read says that it's ideal to do it in one inhalation if you can, but in practice and more realistically it's likely you'll do it in about three.
I use Cook’s extra-long matches to light the pipe. I find that the match can then be kept going. It ensures against the leaf going out, which can happen, particularly if you feel you need to slow or stop your inhalation. Also the match flame adds to the over all heat, which probably improves vaporisation (even if it doesn't make the smoke any cooler).
I use a water pipe that has quite a length of tube on it. Because of the length of pipe, when initially smoking, I take a short puff (or puffs), until the first smoke comes through, then a big exhale before starting a big inhale. This may be the expense of having to re-light the leaf, but it means that the main inhalation does not start with mostly clear air.
Also (again because of the length of the pipe) I find, once I have fully inhaled and held my breath, letting just a little air from my lungs then breathing in again (topping up) from the pipe after it's gone out delivers just a little bit extra [...^]
Remember, deep inhalations.
Also, if you have just got through a pipe-bowl of leaf and did not really get any appreciable effect, it's best not to simply fill another bowl and try again straight away. Salvia's relatively short-lived effect suggests that the brain can quite quickly compensate for its presence. The idea behind deep inhalation is to take Salvia on board faster that the brain can adjust to it. So, if you have just had a fairly unsuccessful attempt, even though you may not have crossed an experiential threshold, you probably still have salvinorin in your system. It's best to give yourself 30-40 minutes for this to clear before trying again.
And try to be clear from the influence of any other substances. I'm thinking particularly of the effect of alcohol, which could interfere, even marijuana, which may not help.
see also Am I a Salvia hardhead?>