My rule of thumb are: Nothing less than an item with a price tag of at lest fifty buck's, never waist you time with the sales clerk - manager is the only one who can curt a bargain, don't be insulting in your price bid, (to low), hot items you will only get laughed at.
Learn how to spot items that have been on the shelf a while and if it's something you need or can use ask the manager how bad he would like to move the item.
Hard nose approach, say an item is on sale for say sixty bucks - take the item to the manager and tell him you have exactly forty, forty five or fifth bucks in your wallet, (amounts all relative), and that's all you got how bad does he/she want to sell it.

I find it both fun and rewarding - what are some of your ideas and experiences?