Haven't been playing too long myself, but here are some mistakes I made early on
1. Buying drivers like crazy
I thought I just hadn't found the "right" disc for me and hitting 400+ was an issue of finding a disc that I liked. I probably spent $100 on drivers before I found Blake's article on choosing discs. A shame that I now have a stack of discs I can't use and rarely throw, but a lesson well learned I can tell you that.
2. Buying primo plastic.
A complete waste this early on. Everything I throw decent on a regular basis is in baseline plastic. Main reason I advise baseline stuff is that when I started I put a lot of discs into the brush. Losing a $6 disc is less painful than losing a $20 one. Even more painful are the expensive ones I still have, but can't throw [see above]
3. Carrying too many discs to the course.
I used to stuff my bag to capacity thinking more discs = better scores because most of the bags posted here have a list of 15 or so discs. So I obviously needed to carry at least 12 right? I ended up dragging around of bunch of junk I threw maybe once just so I could justify carrying it around for hours. I carry about 6 now and can do more with the 6 than I could with the 12.
Some things I found to greatly improve my game
-Practice, practice, practice. I'm playing 4-5 days a week and 2-3 rounds every time I play. If I can't make it out to a course there is a football field a short bike ride from my house where I just go throw for a while. I put my bag somewhere and throw at it, straight shots, hyzers, and anhyzers. There is no substitute for thousands of throws.
-Light weight discs. The first thing I could really drive consistently was a 150g Leopard. I had lost my 175g Leopard [also a great disc] got a 150g to replace it and was surprised how little effort it took to fly dead straight 200+
-Read Blake's articles and read the Technique board. I didn't really get it all the first time through, but eventually the concepts will start to click
Isobar's suggestion is a good one, but he forgot to mention the Comet XD
It is a poor carpenter who blames his tools