I'm only about 360/500 and 70/300 so far.
Few tips to help you find them.
Listen for the orbs!!! When your within about 5-10 feet of an orb you can hear an odd humming like sound. I found a few of these strictly by doing this.
Try to look around when you jump, especially what your jumping over. In the mountains there were a few hidden orbs I hadn't seen the first time and had just jumped right over them.
Think of where you would hide them. Last night I saw a smoke stack in the factory area I was in. I figured there probably wouldn't be one in the smoke stack, but figured I'd put one there if I was a dev, so I checked it and there was one there. They were also in another 2 stacks I found too.
I've still got plenty left to get and they already seem like they are getting hard to find.