If you have the needle in so it cant go farther (shorts) and you rest your hand on your arm you cant get much more steady than that. Part of being steady is your injection hand moving in conjunction with your other arm if you move. If you hand is resting against your arm and the syringe is in a position that it cannot go "in" and farther, only out, you have the most stable position you'll ever get. I watch people that try injecting and they are holding the syringe in mid air with nothing to help hold still or in/out and they fuck up all the time because its impossible to keep that hand steady AND depress the plunger.
I never miss, let me rephrase that sometimes on insertion I wont have the spot, so I pull out, re-insert, that happens like 1-30 sticks. But once I have position and register I never have any problems. Never miss, ever. I've been IVing for about 5 yrs pretty regular the last 2 yrs and in all that time I've missed twice and both times it was because I moved in the middle of the process.
The trick to it is 2 things, learning what angle and how deep you need to go so you get it on the first stick and keeping your hands and arms STEADY from the time you insert to the time you pull out. Most of the people I see that miss are because they dont stay still. Like I said its almost impossible to hold your hand free in the air, not move and depress the plunger. Thats where I see most people make their miss. They register and think they are good, but when they moved from pulling the plunger to pushing it, they move and it only takes a little bit of movement and you're out of the vein and missed shot.
If you use a long tip, you wont insert it all the way in because thats too far with a long tip, which makes holding it in the correct in/out position more difficult