I typically play far too many tables so when I need to quickly figure this stuff out.. I use the 2/4 rule like a lot of you guys. It's more than good enough during a hand. If you want to get picky on the flop, outsx4, subract 1 from each out after 8... so ie. you have 9 outs x 4 = 36 - 1 = 35. If you're just looking for the next street you can just divide outs/unseen cards.

Also something that nobody mentioned.. you have to take into consideration your opponent's redraws (e.g. you have a flush draw they have a set, they also have 7 and 10 'god' outs) so instead of being ~35% to win, you're actually more like ~25% to win. These situations come up frequently on both ends.

Also, sometimes you can count your opponent's cards too (even though they are unseen, sometimes you can deduce what they have or more accurately figure it out from their entire range).