The best and permanent way to make a ring fit tighter is to take it to a jeweler and have it professionally resized. If that is not an option, there are several ways to temporarily make a ring more snug.
Many jewelry counters sell small plastic pieces called Ring Snuggies that fit over the band of a ring to achieve a tighter fit. If the ring is just slightly loose, a few coats of resizing gel or clear fingernail polish will make the ring slightly smaller. Another temporary option to make a ring fit tighter is to wrap masking tape around the band.