Try and pick up some variants, or some mods. When I got bored with Civ a while back, I downloaded RaR and it was like learning a new game. That kept me interested for a bit. When they started to get dull, I tried variants. Always War will make the game more interesting I think.
Now I just play the three in cycles, regular C3C, RaR, and then variants with both. It keeps it new.