Opinions vary considerably on how easy or difficult it is to hitchhike in Crete. I would say that on the whole it is very much easier than it has become in many parts of the world because people here are not suspicious (with the exception of many tourists in rented cars who seem to take their suspicions with them to Crete) and willing to help. What they often don't grasp is the intention of someone sticking their thumb out by the side of the road!
If you hitchhike on a road where there is ample public transport, drivers passing you may not understand why you don't take the bus.
If you hitchhike on a road with infrequent or no bus connections, they will
know that you want to go somewhere and that you need to flag down a car in
order to do so.
This brings up to the next point: flag down and not stand with arm extended and thumb sticking out. Whenever I had no car and needed to get somewhere in a hurry, I was intimating the first car passing to stop. And most of the time it did.
You possibly have to practice the local gesture for this: arm slightly extended and the hand, palm down waving vigorously up and down as if to say "slow down and stop". Don't worry about appearing pushy, you are after all in a hurry and there is no question in your mind that they would not stop. In really out-of-the-way places don't worry about the vehicle seemingly having no space available: space can be found most of the time (how many fit in the cab of a pickup is a loose concept here).
After writing this I have a small requests to those readers who might one day encounter a Cretan doing exactly what I described whilst driving in their rented car: if someone flags you down, they need to go somewhere. Please stop and take them.