Raising the humidity might depend on your set-up. In terms of decor, soil and live plants retain and release water. You've already got that going, so the two other ways to raise humidity are (1) more frequent mistings and/or (2) covering part of the lid with plastic/plexiglass or something that doesn't breathe too well to keep more of the moisture inside the tank. If you've already got ~75%, you probably don't need to raise the humidity further, though.