My own guess is, “By expecting to take a lot of hard knocks.”, but I’m sure there must be a more technical answer!?
Get a coach. Seriously. You cannot learn pole vaulting on your own. It’s not only a very technical event it’s very dangerous,you can’t just bull your way through.
Moved from GQ to TGR.
This is a non answer. What would the coach have somebody do to practice it?
You generally start small. You don’t just fling yourself 20 feet into the air the first time. You learn how to grasp the pole. How to run and set it in the pit. They do at lot of practice without the bar just falling in the mats. That’s what I observed in high school track anyhow.
The only thing the OP can practice on his own is if he has a pole and a sliding box, he can practice his plant. Even that really needs an experienced observer to watch his technique and timing. Pole vaulting is not something that can be described on a message board. I coach high school track(distance) and vaulting is probably the most complicated event to coach. To repeat, GET A COACH. Depending on your age, you might get some coaching from the local high school or college. Most of the drills require pole vault facilities and those are not available for public use.
The pole vaulters at my university would practice in our diving pool. It was deep enough for them to use most of the pole and there was an underwater window the coach could watch from.
So all I’d have to do is hire a coach? Sounds easy. I can sit on my couch and watch TV and be a pole-vaulter! Oh wait, you meant the coach would have you practice? So what would the coach have me do? I guarantee it can be described using text.
http://www.polevaulteducation.org/
http://www.coachr.org/pv.htm
http://www.american-trackandfield.com/read_feature/pole-vault-safety
http://www.antigravitypolevaultclub.com/PV_Bars.htm
I’m seeing a shopping mall kiosk business opportunity here.
I want in! Give me a shout when shares are available! 
Can you elaborate? I’m trying to imagine this, and I don’t see how it would work. Do you mean that the pool was empty? Why practice there rather than on the field? If the pool was full, then the coach was looking through the underwater window and through the water at the vaulters. How could he see anything of note? Also, it seems like the pool would be much worse if someone made a mistake and fell backward rather than forward.
I was a pole vaulter my freshman through junior years of high school. In terms of non-pit practices we did a lot of rope work and step drills. We would use climbing ropes underneath the stadium or in the gym to work on our inversions and form. It was actually an extremely effective way of working on form because you had more time to see how important each step is and the difference between good and bad form.
Another drill involved our poles and one of the empty practice football/soccer fields. We would start at one end and go through our steps across the field over and over. We would do the full run, approach, plant and a small jump straight up to help us further perfect form.
While we were on the pits we would mostly vault without a bar across the standards, using a stretchy bungee cord in its place. The bungee was very forgiving and landing on it never hurt. Other times we would vault with nothing in our way.
When I first learned how to pole vault they taught us how to run, plant and swing into the pit (the mats) facing forward and landing on our butts. This built confidence in the motion and also gave you a small idea of what to expect. In the early stages you are dealing with a pole that is not flexed. To achieve this we simply held the pole closer to the tip and further from the back. Once the coach felt that you were ready to add flex, you would go through drills that just gave you a feel of what it is like to flex the pole. These mostly involved the coach holding you up while you flexed the pole at the end of the runway.
runner pat included some solid sites. I just felt that I might be able to elaborate on certain aspects of the event.
Thanks for all your answers, especially Frosted Glass.
Can you remember how many times you had a bad fall though, FG?
Outside of the occassional instances of me landing on the bar after knocking it down , I was fortunate not to experience any serious results from my mess ups.
When I snapped my pole during practice it was pretty freaky because as you are going through your regular approach you hear a bang and lose control of the pole. I ended up in the mats on my back. As scary as it is to snap a pole, virtually everyone ends up in the mats because your momentum is still sending you forward.
I did witness people hitting the standards (the metal things that hold up the bar you vault over) a few times and a few vaulters ended up off of the mats to the side but these were rare occurrences. For the most part, you could prevent unplanned results by aborting your jump during the approach.