It depends on expertise. One should confirm the location of the vein by applying tourniquet proximally and palpating it before poking. One should directly poke over tumescent vein. If someone can't find the vein even after poking twice, he/she should call for help. In another case some professionals manipulate the needle beneath the skin and able to find the vein. Patients should co-operate. Nurses should do this without causing much pain and discomfort to the patients.