A few tips to stop bruising:
1) First of all, ALWAYS throw away old needles after use, because I swear to fucking god the number ONE cause of bruising during IV is from using a dull fucking point. If you're not sure, just fucking toss it and start fresh...
2) After IV'ing, be SURE to pull the needle out SLOWLY. I swear to fucking god the number 2 cause of brusing during IV is hitting the vein, banging the drugs and having been successful, sharply pulling the rig out in haste, usually causing definite trauma to the injection site, and a bruise.
3) When done, IMMEDIATLEY apply pressure to IV site. As soon as you withdraw the rig, put one index finger on the injection site and apply a bit of pressure. YES, this works very well and is probably the MOST important step to preventng bruising. One of my best friends is a former paramedic and gave me the technical medical explanation behind it once, but it slips my mind at the moment. Next time you go to the doctor's office though, if you'll notice when the nurse takes your blood, after she sticks a cartoon bandaid on the IV site, she applies pressure to the spot, if she doesn't do this, she's not a real nurse, or she's drunk...
4) Try sticking to the inner elbow if possible. The vein located there is larger than most, and is much easier to access overall (allowing for a more controlled IV, less chance of bruising). Rememeber, some spots tend to bruise more easily than others for some people. For me personally, though I can easily hit some veins on my hand, it almost always causes a bruise or mark, no matter how stunnigly perfect my technique.
5) Worst comes to worse, just have someone else preform the injection.
While brusing isn't anything to really worry about, it's not exactly "normal". So keep practicing till you perfect the technique Proper IV admin should NOT cause a bruise normally, or even any real noticable difference at the site of injection, of course this is realtive to everyone...
Just a few things I didn't see mentioned, hope it helped some, have fun...