Hi Fallagal & everyone else. I'm a bit late replying to this but I thought I should share for the benefit of anyone else as well . I work in a hospital and also take blood and pop cannulas in, even for hospital staff it can be difficult, but there are more tools and different types of needles and tactics that we can use to be able to obtain the sample. Obviously dehydration or low blood pressure can make vein puncture extremly difficult, especially if someone is usually difficult in the first place. In general you need to keep warm, try wearing a glove/pair of gloves, you could even wear them for the procedure as the phlebotomist or blood taker will mostly go in the upper arm, but the glove will help to keep ur arm warm. Also with a tourniquet on lower ur arm down beside you and off the chair for a minute and open and close ur fist/hand repeatedly which will hopefully (usually works) bring the veins up to the surface, at this time the phleb or blood taker can search for the vein to reduce the amount of time the tourniquet stays on (as this can cause bruising). Feel around the centre crevess part of your arm for that bouncy vein, it may even feel like a pimple size (which I have found with some people, that's its just a small spot area where a very small part of the vein has come to the surface), the way to test its a vein and not a tendon or anything else is to keep ur finger on that part and release the tourniquet, if it goes down (the vein, deflating feeling) then it's a vein! Other things are filling up a rubber glove or bag with warm water and applying to the area for 5mins and trying again. Also ask if the blood taker has a butterfly needle preferably 23G size, they have a smaller thin needle and with a extension tube (flexible) connected to it that goes to a thing called a vacutainer which u apply the blood taking bottles to. It's great for control of the needle and delicate as can possibly be for ur vein.
I hope this becomes help to any of you out there as I am only trying to help and give the best advice I can from my experience. I don't ever comment on these forums usually.