How do I keep avocado from browning in my salad?

Lemon works pretty well actually, but the problem is probably that you're waiting too long between making the salad and eating it. There's no trick that keeps the avocado perfectly green for 8 hours; oxidation is chemical and it's going to happen no matter what. What you can do is slow down the process and make the salad closer to when you're actually going to eat it.

If you're prepping everything the night before or early in the morning, keep the avocado uncut in an airtight container. Cut it and add it right before eating, with a good squeeze of lemon or lime on all the surfaces. The acid slows down the oxidation, it doesn't stop it, but it buys you time. Another thing that helps is not mixing the avocado with the rest of the wet salad; keep everything separate in different compartments of the container if you can, so there's less air getting to the avocado. And here comes the lesser-known trick: peel the avocado and leave the pit (the seed) inside the container with the pieces. It sounds weird but it really works because the pit keeps releasing something that slows down the oxidation. Try this last one next time and you'll notice the difference.