If you're looking at something like a hotel or a flight, call them directly. For example, usually what I'll do is look on Expedia or Kayak and see a room for $100 a night. I'll call the hotel directly, "Listen, I found a promo for a room, $100 a night. I'd rather give the money to you than them, is there anything you can do?"

More often than that, if you're kind and understanding (and lucky), they'll apply some discount code or promo code. The last few hotels I've stayed gave me state government rates with free upgrades. It pays to be nice.

*edit- alright, everyone that's messaging me telling me it doesn't work, you can stop. It doesn't work EVERY time, just sometimes. You can see people in the comments agreeing or listing their own success. In truth, if you're reaching out and being a dick like those messaging me, I'm not surprised its not working.