Ok, they call you because they can't get an IV in someone. You think, What the hell? They couldn't do it, and I just started so somehow they think I can?
I suck at IVs, I sucked at them for a long, long time. Lately, it's been going better. One day it just suddenly got a lot easier.
But for the truly hard stick - here's my favorite tip. Get a towel and run it under hot water (not too hot to burn, but hottish).
Put the hot part of the towel on the place you want to stick - antecubital, hand, whatever. Leave it there for a few minutes.
This causes vasodilation - often a good vein will pop up, and if not, a bad vein will still be easier to stick.
It has the added benefit of calming things down a little. The patient has been stuck probably several times by an increasingly irritated nurse, and is probably increasingly irritated himself...the time that the towel sits there gives a little break for everyone to relax.
And - when really screwed - you can get a tiny IV from pediatrics. Unless you are in a serious resuscitation situation, it's probably fine. If people bitch about this, tell them that smaller IVs allow better blood flow around the cannula and are less likely to get infected.