I don't know whether you have had this system in the past or would consider adding it to the site but in my experience it can really encourage some of the more involved and dedicated members a bit of extra encouragement to be active and posting.
There is many ways you could reward a member of the month, from giving them RG XP, a free month prime, cash reward, or a custom usergroup for the next month. Down to you.
Suggestion for if you have an additional usergroup for the CSS design:
background-image: -webkit-gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) ); background-image: gradient( linear, left top, right top, color-stop(0, #f22), color-stop(0.15, #f2f), color-stop(0.3, #22f), color-stop(0.45, #2ff), color-stop(0.6, #2f2),color-stop(0.75, #2f2), color-stop(0.9, #ff2), color-stop(1, #f22) ); color:transparent; -webkit-background-clip: text; background-clip: text; font-weight: bold;
This makes the username look like this: