First thing I do is ask myself what I want a disc to do. Go left? Right? Long? Short? Approach?
Then I look for a disc that will meet that need, and talk to people about what they might use for that type of shot.
After I get a disc like that in, I go to my practice field and make 50-100 throws, and watch the disc carefully, and study how it flies, how it fades, or turns over, and how far it goes. I also see if I have something in my bag that its going to replace, or if it is going to go in the bag at all, or back up another disc in case I loose one. I do this will all my discs.
I also write the info on the rim, so when I look into my bag, I know what disc will do what for me. Its a great way to find just the right disc for the shot. Also, some discs can do double duty , like your disc for hard fades, can also be the best disc to throw into a strong headwind.
I like to know my discs backwards and forwards, before I ever hit the course with them.
I have several discs coming in right now, and this weekend I will spend 1-2 hours in the field with them learning how they will fly for me.