Everyone who has started more than 1000 IVs in their life raise your hand.
OK, with that out of the way, some thoughts.
@jon-nyc , the back of the hand is certainly not the best place to start an IV from a patient's perspective. It restricts motion, is easily dislodged, etc.
But...
It's frequently the easiest because the veins, though not the biggest, are usually the most visible. Most visible makes it much more likely that you'll have a first-pass successful stick. Most successful means most comfortable. If you need to stick someone 4-5 times, so that you can have a more comfortable site elsewhere misses the point.
Always use local anesthetic for anything bigger than a 20g iv. The numbing makes it easier for the patient to tolerate you"rooting around" while trying to find a vein.
The ideal location is the forearm - it's between two joints, and it's stable. However, unless the veins are really easy to see, good luck. If you can feel them, good luck too. Ideal place on the forearm is at the wrist, just this side of the thumb joint - about where you feel for a pulse. There's usually a vein there that's pretty easy to hit, even if you can't see it. You should be able to feel it.
The problem with missing a vein in the forearm is that is some cases, you only have that one arm to work with, so if you blow a vein, putting an IV closer to the hand might cause the injection to leak out of the hole you made earlier, which is upstream from the hand.
It all depends on what the needs are for the IV. For longer-term things, forearm is absolutely the best. But for a lot of surgery where it'll only be in for a day or so, use the hand, put it on an armboard. For really short stuff, where the IV will be in for an hour post-op, I got to the point where I just put them in the antecubital fossa (the elbow, where they draw blood). Patient wasn't going to move, if I did my job right, and afterward, they would be good to not bend the elbow for an hour.
If you're feeling adventurous, put it in the internal jugular vein.
I'm sure I have more thoughts - but I just can't think of 'em all at the moment.