I like to keep implied odds calculations simple, and the most important thing is guestimating how much value you can get from a made hand if you hit.

For our intents and purposes, assume the following:

There's significant money behind.
Villain is competant and has a huge hand (ie nut flush)
We are behind.
We are drawing to the nuts.
Villain isn't going crazy on the river with a second best hand.
We are drawing to 10 outs, and we are on the turn betting round.
If we make our hand, it will be checked to us, as our villain can see the board just as well as we can, and he knows his hand is now very vulnerable.

Numbers are just off the top of my head, but seem reasonable.

10 cards make our hand, 36 do not.

$100 in the pot. Villain leads $50.

Further assumptions: if we call on the turn, and hit on the river, villain will check to us and call a pot-sized bet exactly one half the time on the river. This is much more relevant then the long list of assumptions above. This is exactly $100 in value on the river, if we hit, according to our assumption.

So... he leads 50 into a 100 pot, that's 150, add the 100 in value on the river, 250, and it's 50 to us. We are getting 5:1 in implied odds here and are only 3.6:1 against hitting. Clear call.

Nifty eh? I try to keep the assumptions as simple as possible, and these kind of things have run through my head so many times it's just automatic to me now.

edit: just note that we are only getting 3:1 in expressed odds which isn't enough, but implied odds are very important in NLHE, particularly (in general) against players who take second best hands too far.