This brilliant Google Script periodically goes through your Gmail account and labels messages between 5 and 14 days old where you were the last person to respond. It's a perfect way to make sure that you follow up when your recipient fails to write back. And you don't need a third-party service to do it, since the script runs under your own credentials.
Speed up Chrome with FlashControl extension
Recently, Chrome started really dragging on my Mac. I opened up the Activity Monitor and figured out that it was Adobe Flash that was taking up all my CPU time. If you're having this issue, try installing the FlashControl extension for Google Chrome.
6 Easy Tips to Speed Up OS X Yosemite on Your Mac
OS X Daily has a great post out about how to speed up OS X Yosemite on older (and newer) Macs. Step 1, "Disable The Eye Candy Transparent Windows & Effects", has done wonders for me. Animations in Mission Control and swiping in full screen mode are much smoother.