Something that's helped me lots is this: once you have blood flashback, as someone else said, STOP. Reach up and untie the tourniquet with the other hand. Then I don't put the needle any further in unless it's some huge vein. I put the whole thing lower down to the skin - this usually involves taking my index finger out from under it- and with my other index nail, yes it's short but there, I advance only the catheter. I was about to say "only the pink part" but they are different colors. I'm just very fond of 20's if I can get 'em in.
Then if it doesn't stick going in, I'm good and can attach saline, aspirate and flush, etc, and finish up. If it sticks going in, I hold it right there, attach extension set with saline, and gently push saline ahead of the catheter and attempt to float it on in. If there is a chance at all, that'll help, such as on dehydrated people. It's only worked for me once, I know someone for whom this works all the time. My luck is such that if I'm gonna be trying to float something, they move.