Fashintern, as far as tips persistence and frequency is my best tip.
I have a goodwill very close to my house, so it helps to stop frequently. I also prefer a local independent thrift store because prices are better and quality is better (they weed out really ratty stuff, whereas GW does not). Getting familiar with brands and even the feel of a good fabric helps make it easier to browse. (You guys are probably already past that) Ask your store when/where/how they put out new stock. At my GW, they put new stuff on the inside aisle and I often find multiple "treasures" that have just been stocked! Persistence is key, plus being totally willing to shop out of season. This is a great time to look for jackets and sweaters (spring is even better when people are clearing out their winter wardrobe).
I believe all GWs will now accept returns with receipt and tags. My goodwill just applies plastic tags (no paper tag anymore) to their clothing. SO what I have realized is that if I find something awesome with a stain, I will take it home and wash it. If I get the stain out, I keep it, if I can't, I will return it. I have no guilt since I'm returning it cleaner than I bought it. I've only done that a couple times but is nice to have the option. I don't think people try very hard to get stains out! Oxyclean really works!