1. I look for things that fit perfectly right off the bat. This is more rare than you get in a regular store with standard sizing. I have to be in the mood to thrift shop because it can get discouraging to have item after item not fit. Look for something that fits like a glove and it makes the decision to get it even easier!
2. I look for items that I like and can be easily changed. Is the pattern on a skirt great but it is just too long? Is the top really cute but just a bit wide? If it is something that I know right off the bat how to fix, I will buy it. I end up with a mend pile but that's ok with me. :)
3. I will not buy something if there are things I can't fix. This may be a no-brainer to you but this took me a while to figure out. I would buy anything and everything and never fix it. I have a really cute robe type thing that I thought "I'll just shorten it, take off the sleeves, add elastic, etc" Still hasn't happened. This also applies when the garment is ripped, stained or has holes. Even thought things are less expensive, if they need a lot of work, leave them behind.
Take the 3 shirts below.
My favorite is the blue and white checked one. However, it was too wide and the poof on the shoulders was way too low. It looked huge. I could take in the sides but I have no idea how to fix the sleeves. So, I left it there. The pastel shirt was wide at the top and a bit tight on my hips (story of my life), so that got left behind there as well. I ended up getting the shirt on the top left. Do I need another plaid shirt with a navy base? Nope but I liked it and it fit so I picked it up! It has a lot of colors that aren't in my other shirts so that was enough for me to pull the trigger.
4. Check the tags. This is where I contradict myself. I don't normally check the tags before I try stuff on but I do check them before I buy. What is the item made of? Is it worth the price that's on it? Is it vintage or just a new shirt pretending to be vintage? Does it have a cool brand that would be fun to brag about?
If it is a good quality material, vintage or a brand name I recognize... I will pay a bit more for that. If it is lower quality material, new,etc. I will put some of those away if they are priced too high. For some reason, the thrift stores seem to be charging more for "vintage" these days. Also, things from Target seem to be more than if you got them on clearance. Thrift store pricing can be baffling so just keep these things in mind!
Hopefully this information has been helpful! Let me know if you have any questions!