You phone in your order and tell them that music123 is offering 15% off with this code 15off4u. Make sure that music 123 carries the same item and they will beat the discount by giving you 20% off. They are aware of the competitor's discounts and product offerings. Check out the other online retailers and you may find even bigger discounts which they will match or beat.
I do all my buying I possibly can from one guy, a sales rep at sweetwater. When I want something, I email him and tell him what it is and ask what price he can give me. Because of our ongoing sales relationship, he gives me great deals on everything, so I stopped shopping around. That's the way to do it, imho.