You just have to count to three and do it. Stack up the beam or ask for spot. You could have your coach just stand there on some. Also, start on a lower beam and work your way up. Hope this helps. Good luck.
something that really helped me was to go on whatever beam you are comfortable on even if it is just a line on the floor and do the bhs but jump so high (not back but up) that there is a definate second you see the beam upside down before your hands hit it. This made me feel more comfortable because i knew if i was a little off, i have that split second to change course. Then just keep looking for the beam before your hands land on the beam and you will feel more comfortable.
if you mean throwing your back handspring on the beam, then listen to what these people have said.
but if you just mean throwing it on the floor--then there isn't that much else we can tell you.
if you know how to do one and your coach tells you you don't need spotted--then you're just going to have to GO FOR IT.
it's scary, but it's gymnastics. And once you do it, it'll feel great to get over your fear.
what's the worst that can happen?
i've seen a loooooot of girls fall on their heads doing back handsprings and getting up perfectly fine. They didn't even know they did anything wrong.
so don't work yourself up too much, okay? think of how great it will feel after you do it [:
If you really can't go when you get up on the beam then i would definitely practice on a line on the floor first. Throw your BHS on the line and use the proper hand placement on the line just like you would on the beam, also watch for your feet land on the line just like on the beam. On the line you can also work on throwing at the proper height so you can see your hands as you put them down. (like dancing9118000 said)
For hand placement i do one hand in front of the other. My (left) hand is near the space where the " L" shape from the thumb and index finger of the (right) hand is...if that makes sense...i think this is the common hand placement.
It's probably an issue of just getting your confidence up.
After you are confident on a line move up to one of those beam pads, but use it on the floor (the ones that are meant to go on the beam to make it wider) lol sorry i forget the name cause my club calls it "the road".
When you get confident with that move up to the low beam (you can even put mats beside the beam so it's lower if you want). Get your coach to spot you on the low beam too. So basically start at the easiest height that you are comfortable with, move up a bit until you are confident to move up again. Remember your coach is there to spot you and won't let you get hurt.
that is EXACTLY how i am!!
i will do it on the low beam, but i won't even do it on any other beams even with a spot!!
so i can't really help, lol, my coaches just tell me that ik that i can do it, i just need to go and that i have it.