Welcome to Bluelight
I think your best bet is to stick with the veins in your arms, but get very good at your injection technique, so that you leave no bruises or scars.
This is fairly easy to achieve, providing you follow some simple steps, such using a new needle for each shot, as well as applying pressure to the site immediately as the needle is being withdrawn. A cotton bud does the trick fine, but be blood aware.
Also, make sure the needle stays very still inside your vein. Some finer guage needles (29g B&Ds are known for this) can barb when people are mixing up. You may not be able to see it, but the very tip of the neelde can get bent backwards pretty easily. Avoid applying any presure to the needle whie you mix up, because a barbed needle is a guaranteed way to end up with a nasty bruise.
Also, after your shot, don't bend your arm when applying pressure to the site (like some nurses still do). Hold your arm staight up in the air, applying firm pressure for about 5 minutes. This feels like a long time, but the longer you apply pressure, the less bruising.
Also, if you bruise easily, you can then apply an ice pack to the site to reduce bruising even further.
Vit E cream etc also provide another way of helping the site recover but apply this a few hours after the shot, so as not to irritate the site with the chemicals in many of those Vit E cream preparations.
Various kinds of makeup can also easily mask minor bruises. If you are bruising very badly, you need to look at your technique.
The reason I am suggesting your try to stick with the veins in your arm is because many of the other veins commonly used when trying to be stealthy can be quite dangerous.
Don't inject into your feet, toes, or ankles, as these often have fungal infections and odd bacteria hanging around, significantly increasing the chance of things like endocarditis or septacaemia.
The veins in your feet are also very small, meaning the blood flows slowly through them. This gives infections more time to develop. There is also greater risks of vein damage when using these small veins in the feet. Although it is one method that is easily concealable, the health risks are too great. The complications from this can be very serious.
Injecting into the groin is also very dangerous, due to the close proximity of the femoral artery. In some people, the femoral artery is tied around the femoral vein, meaning it is virtually impossible to safely use this method. If you are successful at hitting the femeroal vein, there is then an increased risk of deep vein thrombosis, and more difficult to treat infections. This site should also be avoided.
Another secret place people sometimes inject, yet also extremely risky, is the back of the legs. Blood flow through these veins is also relatively slow, meaning it is harder for the body to fight off any possible infection of the injecting site. It is also very easy to miss the vein by injecting too fast into these veins, as the blod flow is very slow. Don't try to inject into varicose veins either, because they will bleed very badly.
There are some other sites that people use, however these sites are absolutely dangerous, and should never be attempted at all. These include the neck, breast and penis (yep...it does happen). These areas carry very high risks, and should absolutely never be considered.
There are some veins on your upper arm which can be covered by a regular shirt sleeve. You may not be able to see these veins initially, but flexing your muscles, swinging your arm around, applying a warm towel to the site, or even having a warm shower can help raise veins that you may not otherwise see.
Just be aware that stealthy injecting can sometimes come back to bite, when you end up with a really nasty abscess or something.
Good luck

[ 15 March 2003: Message edited by: Flexistentialist ]