There have been so many times when I NEED a piece of photo equipment only to have it sit unused in my camera bag . I usually realize my mistake right after the 30-day return policy expires.
On the other hand, I have also purchased gear that has become worth its weight in gold. Here's a few examples:
Manfrotto 166 Utility Apron - I can't imagine not having this apron on my tripod. It is a catch-all for lens caps, filters, batteries, my phone, lenses...anything. It's a brilliant use of space and you can even load it up with rocks for more stability.
This small invention single-handedly killed the canned air business. It works great, blows dust from every crevice and doesn't end up in a landfill.
Plug it into your camera, download the app and your images are instantly transferred to your iPad. The only people that love this more than me are my clients! Nobody wants to stand cheek-to-cheek looking at your 2-inch screen. Plus, besides seeing the photos, you have complete control over shutter speed, aperture, ISO, white balance, focus and can remotely click the shutter or watch Liveview.
Hope you you like the tips, I'll add more soon. What's your favorite gear?