James is right...there are days that you feel like you couldn't hit the broad side of a barn, then others when you get a vein that seems to come out from nowhere.
But I agree, don't pull the cathlon out immed. if you don't get an initial blood return (this is part of that mantra I mentioned earlier). About the only time I'll take one out right away is if the pt complains that the site is very painful (for some, the wrist area is too tender).
Another thing I forgot...a lot of people will sort of "slap" the area where they are going to stick, trying to raise the vein. This will actually make the vein shrink down. The best technique I've tried, and use it all the time, is to just very lightly tap the area with your fingertip. You would be amazed at the veins that pop up by doing this. Another is to stroke the vein very lightly in the opposite direction of the blood flow.
And one thing that wasn't mentioned...choose the appropriate size cathlon. A #20 or #22 will usually be sufficient, but in extremely difficult sticks, like people whose veins are sclerosed or just have "SFV", I've used a #24...we were still able to run fluids at 125cc/h, and I got the blood for the bloodwork with the IV start.
For the very anxious pt, the stress can make your vein vanish just as you're getting ready to stick, so I've used guided imagery with some adults and older kids, had younger kids blow soap bubbles while I'm sticking, let the kids try to pick the site, etc. Lots of hugs and + reinforcement when it's over.
One last thing...pts who seem to get a perverse pleasure out of trying to jinx you ("You'll never get it...the last time I needed an IV they had to stick me 8,456,293 times...I have terrible veins.") I love to just smile and say, "Well, then today is your lucky day, because I'm the one that is going to be starting your IV." Don't let them try to psych you out...tell them how good you are (even if you're not the greatest yet, keep telling yourself you are doing great and are getting better every day...a little bit of confidence goes a long way in being able to get an IV).