A few that would top my list (in no particular order):
1. Not knowing your limit. You have to know how much you can spend before making any system plans. Try everything first together in the exact set-up before you buy and make sure you like it or swap an amp,source, can or cable before you commit to anything and end up having to pawn it on BST and losing 10% of what you paid in the process.
2. Don't be intimidated by others. Yes, some people do have 12 000$ Orpheus systems. Get over it. Most people don't have 12 grand to spend on headphones and the like. Find the best possible set-up for you (within budget) and stick to it.
3. Don't always go for the biggest and best when they come out. There is such a thing as impulse. Maybe you bought E5cs just before the E500s were announced. If you like the E5cs though you don't necissarily need to upgrade. The double the price doesn't mean double the performance, and that may be 200-250$ that is better spent on something else (like an amp
4. Don't take people's advice only before buying anything. Try to make an effort to try them yourself. Find a store or meet near you with what you try and make sure you like it. Impressions are good, but when putting down a wad of cash for cans you most likely can't return, it's best to be 100% sure.
5. In the end it's not about if your system is a 1000$ GS-1000 or a 10$ KSC-75. Find what works for you and stick with it. The best can or source or amp may not be the best for you.
6. It's easy to get caught up in the equipment. Remember: it all boils down to the music! Buy, listen and enjoy.