If I haggle, I use cash, and say I have $20 what will that buy me. Haggling can make it fair for everyone. Don't screw the LFS, but if you can buy a fish for $1 at a chain (neons what & nots) I am not paying $3.99 each at the LFS. Usually I have a fair price in mind, and I don't mind paying a few extra dollars, especially if they are easy to deal with.
I have a LFS that is less friendly, but huge higher end stock. I usually don't buy because seldom offer good deals.
Or
A local start up that bought my dry live rock gave me store credit. Offers better deals for fish and plants. Friendly owners. I try to throw them as much business as I can.