I'm not sure how easy it is to actually miss the cavernosa -- it seems a bit like shooting a barn door at point-blank range. If you have some accidental twist, and you are aiming at three or nine o'clock, I don't think it should matter much. But even so, I still watch out for this.
To add to James' points:
* With the correct needle length, push the needle right in to the hilt. If you feel some extra resistance at the "right in" point, the needle may be too long and going out the other side of the cavernosa.
* Make sure you inject all the fluid -- if injecting manually, push the plunger fully home.
And some things that I am going to try further, without knowing how effective they are:
* After injecting, do some Kegels to help the penis pump up -- my urology nurse said that Kegels done well are as good as Viagra -- and cheaper too!
* Watch the position of the hole in the tip of the needle. If the cavernosa is like a sponge, and you inject at its center, I don't see why the needle hole facing forward, back, up or down should make any difference. But just the same, I'm going to start testing that.
* Try a constriction ring. This need be nothing more than a rubber band wrapped a few times around the base of the penis. A bit of a balancing act because you don't want to restrict the incoming blood, but at the same time you want to keep the drug in contact with the "smooth muscle fibers" for as long as possible. Does anyone know where smooth muscle fibers actually are -- in the penis itself or inside your body at the penis base? If it is the latter, then the constriction ring may be a bad idea.