Practice is the main thing, but there are a few "tricks". Before you start with the failed attempts, make sure that your mirrors are set correctly. You should be able to see a sliver of the side of the car in addition to the road next to and behind the car. Next, figure out exactly what you can see. The easiest way is to have a friend slowly walk toward the side of your car and you identify when they are at the bumper. You'll find that they are a lot further than you think if you can see them in the mirror. Next, set up some cones, or trash cans, or even just use some parking space lines, and a friend to practice going in and out of a parallel space so you get more of a feel for the space around your vehicle.
One of the biggest things is not to let it stress you out. In the military, vehicles have designated drivers who are professionals at driving in combat areas and when parking in base camps, many vehicles require "ground guides" to park them because military equipment is expensive, and fender benders add up, especially when accidentally backing an amphibious assault vehicle over a Humvee.
One last consideration is your vehicle. When I had my Dodge Ram, I didn't even attempt parallel parking unless it was at the end of a block. It didn't turn well, it had limited visibility because of the lift, and it was larger than most spaces are designed for. If you have a compact car, get it into a space and look at just how much room you really have.
Hope some of those helped. Good luck on your driving test.
I once accidentally backed an amphibious assault vehicle over the front of a Humvee while parallel parking it.