The best technique in identifying an appropriate vein site for IV catheter insertion into the arm is to apply a tourniquet to the selected arm 4 to 6 inches above the proposed site.
It is very important to use the tourniquet, because without it, veins aren’t so noticeable. Putting the tourniquet on makes the veins pop out. Also, it is very important how you put it: if it is too tight, the circulation is cut off and there is no blood in the veins to make them puffy., if it is too loose the blood continues to flow and won’t pool for the puncture.