Went to our local builders merchant yesterday to get some paint, and as they didn't have the right kind in (despite telling us on phone they did) they offered us a discount of about 35% off the next size up, which made us very happy 
We have bought a few things from there before and occasionally get a small amount off if dd's swimming teacher serves us, but never this much!!
I'm assuming he gave us a trade discount, but how do we get that all the time?! Do we need to open an account, haggle every time we go in, or is there some other trick we should know about?