Shop around to get good deals. You can’t just go in and say, “Give me that for 50% off.” You have to hunt. And, you’ll get better at it as you do it more. Ask more experienced musicians if you’re getting a good deal. The salespeople will always tell you that you are.
I recommend developing a relationship with a nice salesman that’s been around for a while. This is hard at many stores because of the high employee turnover. Most employees get a job at a retail music store so they can get the coveted employee discount. (To get stuff for the band, man.) Find out with whom you’re dealing, ask how long they’ve been working there and in the industry. The people who have been there for a while will stick around longer and will be more knowledgeable. By dealing with the same person, you will have an easier time and will be less prone to getting ripped off. They will realize that you’ll come back to them and they’ll treat you right as a result. They will remember what you’ve already purchased and will know what goes well with what you have. It will also make returns and exchanges easier.
Buying online can be tricky so make sure to read reviews. Things like accessories shouldn’t be a problem. If you do buy online, check the return policy before you check out! Some places don’t take returns at all!
Knowing all this should save you quite a bit of dough and headaches throughout your musical career. Good luck with your shopping.