I never use a tourniquet myself. I just rely on knowing how far a given needle length has to penetrate into which vein, although lately found that a decent way to get a vein to stand to attention, so to speak, is to use an elastic hairband. Never blown a vein, but it can't quickly be taken off when IVing.
Always push the plunger slow and steady, depending what you are injecting that could save you from an OD, if doing so you realise you prepared in excess of what you wanted, in addition to being less likely to damage a vein. The former being less important to me personally, as I shoot methoxetamine, rather than H and the dose is not nearly so critical as it is with heroin in terms of 'oh shit I just died' capacity.
Best angle I've found out is a bit short of 45 degrees (I think thats right, I am AWFUL at anything involving math, dyscalculic, gah, but I raise the syringe around an inch and a quarter high from a position parallel to the arm at a distance away from the needle entry point roughly equal to the length of the distance from forefinger tip to knuckle), sometimes steeper just to allow easier penetration and as soon as it does so, then immediately make the angle shallower to avoid going through a vein.
Bruises themselves are nothing to worry about, a perfectly safe (well, at least in as far as IVs go) shot that goes in a vein as it should can still bruise. Bring poked with a long, sharp stick can do that
I find that a 30g needle is MUCH less likely to bruise. Only got a few of them left, and they are not easy at all to hit deeper veins with, as mine are only 0.5 inch, very little bruising compared to my one inch 25g needles, although its easier to slip out if one's hand is unsteady, with a 30g, and not as easy to register. I do prefer 30s though. Its just a matter of skill in IVing as to which is best. If you are good enough to hit and stay there with a 30, use them, they are less painful, leave less track marks and are kinder to veins) I think I might get myself some 28g longs next time I purchase needles.
Sometimes its hard to register with a 30, due to increased difficulty sucking up solution, or by extension, blood through the narrow bore, presumably more so since I take tizanidine, a clonidine relative, and that, like clonidine, is a hypotensive agent, meaning I have low blood pressure.
I suggest also, Mr.suicide, in investing in a few things, 0.2 micron filters (exchangesupplies.org do single use filters cheaper than I have ever seen anywhere else, very, very cheap, a low-deadspace model too which is even better), for keeping both particulate shite, and bacteria out of the shot, always use a fresh, sterile needle and syringe for every shot (only time I reuse a needle is to shoot, then to shoot again immediately afterwards, without delay of more than a couple of minutes or so while the remainder of the solution that would not fit, or a second shot is prepared and filtered, that I consider excusable, I see no need to replace the syringe and needle for use no.2 of a given needle, if it is for IMMEDIATE use then and there without delay)
Also, sterile water amps, they do 2ml ones cheap as well as little sterile water plastic pods. And, last but not least, an amp cracker. I do not need one, but they give them free with every order of 10 ampoules of water, basically little plastic foldable gizmo that fits the body of an ampoule in one half, and the head of it in the other, a quick, sharp snapping motion breaks it neatly and cleanly across the neck of the amp without the risk of a glass cut.
I'm practised enough at doing it myself and never cutting myself, but they ARE handy.
Edit-bacteriostatic water is NOT bactericidal. It prevents growth of bacteria but does not kill those present. Do not inject bacteriostatic H2O by itself, this can cause vein injury. Only needed for storage, not for preparing and using single shots or for those used rapidly.
H will hydrolyse to morphine (and afaik monoacetylmorphine) in water, so is best not stored long btw.