when in doubt bet it as you normally would, say 1/2 flop, 1/3 turn 1/4 river, whatever your normal pattern is
however, the ideal time to flop nuts is when you are defending your blind (let's say) vs. an aggressive player ... let him c-bet into you, smooth call, let him fire another bullet on turn
then if the poker gods are smiling on you, maybe the villian makes a nice hand on river and ships you all of his chips ... woot woot
and as another poster pointed out ... one good way to get paid is to have got caught
bluffing a few times or if you have been aggressive in raising pots, which can plant the seed that you are playing less than premium hands or are inexperienced and are over-valuing your hands
finally, if you can put your opponent on a hand, you can exploit the situation better ... i know, why put an opponent on a hand when you have nuts, but it can help you determine what to do ... i.e. you could c-bet if you raised ... a lot of players love to call c-bet, so then you check the turn like you got caught bluffing, they bet, you smooth call ... then maybe you over-bet the river to feign another desparate bluff attempt ...
bottom line is having the nuts is a good thang and usually fun and profitable