The vein along the medial side of the wrist, just below the thumb, is awesome for me usually -- sorry, the name of it escapes me. It's often very palpable even without a tourniquet. I've heard it is called "the dummy vein," because any dummy can hit it.
I will first look to place a 20g, but that's just not always possible. On my floor at least, we use a 24g as a last resort. I hate using the AC personally, especially if the patient maintains a running IV, simply because every little bend of the arm means endless beeping.
Another tip I have found useful: on those fragile, elderly veins that blow if you even look at them funny -- instead of using that tight tourniquet, use a BP cuff inflated to about 20 mmHg above the patient's normal systolic. I have found that the veins will seldom blow that way. And when you see that first flash, hold steady and don't move to advance the catheter until the flash moves down the tubing about halfway.
Works for me! Hope that makes some sense. And practice, practice, practice!