I have a shopping rule that has saved me from buyer's remorse, it works 99% of the times guaranteed: If you see something you like, but are hesitant to buy it for any reason, wait at least a day before getting out your credit card. If you're still thinking about it the next day, go ahead and get it.
Another tip, if it's a sale or one of a kind, check the store's return policy. If they're easy on returns and will give you your cash back (not store credit), buy it but don't take the tags off! Wait for a while and if you still like it after a couple of days and can't wait to wear it, feel proud you made a great buy! If not, no problem, just hit the returns desk.
I'm also a serial online shopper/stalker. I'm suscribed to all of the maling lists of my favorite stores, like Saks, J.Crew, and anthropologie to always be in the know of what's new. If I see something I like, I then add it to my cart, and wait for it to hit a sale to buy it. If you're suscribed to their maling lists, stores will always send you an email when your item goes on sale (Shopbop is great at this), and you'll be the first to know about discount codes. This has saved me hundreds of dollars. I get my stuff delivered to El Salvador through Trans Express.
I leave you with the things I'm coveting right now, I've got my online stalker cap on! :)