Free: Contests & Raffles.
Wind, Shadows, birds, etc will trigger cameras. I make sure I point my cameras facing north if at all possible.......never south into the sun. I cut down brush and branches if at all possible. Make sure your camera is on a big enough tree that it will not move in the wind....triggering pics. If you camera has a sensitivity setting, change that. Grass, has always been the worst culprit in my experience.
Get it up from the ground and get rid of all the wigglies. CHange sensativity, and interval. Also, if you have a low end cam, trigger speed and recovery time may give you issues. Read the manual. Trailcampro does great reviews and lists all specs for comparison if you dont already know that. Not suggesting you havent already done whats necessary, just a reminder.......