For those of you doing IVs, this video is one of the more enlightening that I have seen. Most text & online sites will suggest a 15-30 degree angle. That is misleading! When going into a vein, one needs to go IN-and-Up, not lift needle up and go down! Keep the needle flush (having or forming a continuous plane during insertion) with the vein site, gliding into the vein.
https://www.youtube.com/watch?v=9Mtzrxxgt1sAttachment:
butterfly_needle.jpg