Thanks Spoon. I have always struggled counting combos so this is really useful.
Are there any similar methods for quickly counting the different ways that an opponent could have 2 cards of a particular suit in the case of flush draws? The best I've come up with is:
Using spades for the example ((13-#of spades on board)*((13-#of spades on board)-1))/2
Not quite as easy to swallow!