Build berms or hill up plantings. Since the eye doesn't see what is behind a hill, the mind will imagine there's a greater space. Golf course designers know precisely what this is about. They hill up sandtraps and put the green behind it so the golfer doesn't know exactly how much room there is between the sandtrap and green, forcing golfers to guess. Reds and yellows jump out at you.
Dull colors recede.
Once again, the mind feels more depth. Build an arbor or arbors to separate areas of your yard or garden. Attach a mirror the size of a gate to a fence or at the end of a pathway. It will reflect your garden and make it appear there's more garden past the mirror. It's relatively inexpensive and creates a real hoot as visitors try to walk into the illusory garden.