You've been told to practice so I'll refrain from echoing that sage advice. I have to ask though, as a driver of many years I find it incredibly lame for people to back into a spot in a public parking lot if it is not required or necessary. No offense meant to people who do, but if it's not necessary then why bother?
But yeah, practice.
I park ass backwards 95% of the time. You'd hate seeing me drive.
And I don't know if practice is what you need. You need someone who knows what they're doing to show you how to do it. My stepfather was sitting beside me one day (I already knew how to drive), and he just gave me simple advice on how to back into a spot. I tried it, it worked perfectly, and it has been very very easy since then.
Follow my little diagram:
1. Drive straight.
2. When you see an awesome parking space, drive towards it.
3. When you get close to the parking spot, turn your car the other way and drive away from you. Essentially, once you get close enough to the spot, make a semi "U-turn".
4. Stop. Look behind you. Change gears to REVERSE.
5. Back into the spot.
When I'm at step 4, the back of my car is facing the parking space directly. If I straighten my steering wheel, I can almost back straight into the parking space. The car will require a slight adjustment in angle. That's all. Easy.
If you don't do Step #2, it'll be a lot more difficult to back into the space because you need to back your car into the spot in the way that's similar to driving forward into the space. This requires you to turn your car by 90 degrees.