Are you talking while its on or off?
While its on the internal fan is plenty to keep it cool
While its off you have two choices, either turn off WiiConnect24 standby so that it builds up absolutely no heat or else get a stand with an intercooler or something like that.
I personally shut off my wiiconnect24 standby since more often than not I just use the wii for playing games (which obviously dont require wiiconnect).
BTW in case your not sure, your WiiConnect standby is on if your Wii has an orange light while its off. If it has a red light while off than the standby is disabled.