You've found a fit with your partner: you have at least 8 hearts between you. But how high should you go? Can you make eight tricks? Nine? Ten? Twelve? One guide is your combined high card points, but in a suit contract points aren't everything: shape can be just as important.
Enter a weird little gadget called the Losing Trick Count, which turns out to be a remarkably accurate guide to the best contract with your chosen suit as trumps.
Weird? Well, yes. You use a pretty rough and ready method to work out the number of 'losers' in your hand, and partner does the same. You then add your losers together, subtract the result from 24 and ... Bingo: you're left with the number of tricks you can make in that suit.
So how is it done? Let's take each part separately: first counting the losers, then counting the tricks.
0, 1, 2 or 3 losers in a suit?
It helps to imagine a suit being led three times in a very standardised way: the Ace takes the first trick, the King takes the second and the Queen takes the third.
Take a holding in your hand  diamonds, say. If the suit is played as just described, how many of the diamonds in your hand will win a trick and (more to the point) how many will lose one?
If you have a void, or ♦A or ♦AK or ♦AKQ the answer is zero.
With ♦A4, ♦AKJ or ♦AQ8, you have just one loser.
With ♦K5 you have just one loser, too. As you do with ♦AK654 (any card beyond three is disregarded). ♦AQ and ♦KQ count one loser too: in each case, both cards could make tricks in a longer suit, but as they're in a doubleton, you'd only expect to make one from each. The same goes for a singleton ♦K: it's a high card but it's a loser because when the Ace is led it'll drop under it.
What about two losers? All of these qualify: ♦A53, ♦KJ8, ♦QJ2, ♦94, ♦Q4 and ♦K1097653.
And these weaker holdings count as three: ♦432 ♦J109, ♦J1098, ♦87543 and lots of others!
Note If you want to get sophisticated (which you should, eventually!), the 'unguarded' Queen – e.g. Q32, Q87, Q953 – is a special case. You can only count it as 2 losers if you hold an Ace somewhere else in your hand – otherwise it counts as 3. 
Counting losers in a hand
Once you can count a suit, counting the whole hand is easy: you simply count losers in each suit and add them together. Let's have a go. How many losers do you have in this hand? (Tripleclick to see the answer.)
♠A86 ♥KQJ109 ♦Q3 ♣J83
The answer is 8: 2 in spades, 1 in hearts, 2 in diamonds and 3 in clubs.
How about this one?
♠K3 ♥A54 ♦AK1087 ♣Q54
This time the losers amount to 6: 1 in spades, 2 in hearts, 1 in diamonds and 2 in clubs.
Suit contracts only
It's worth repeating that the Losing Trick Count only helps in suit contracts (once you've found a fit with partner)  not no trumps. An example will explain why: imagine you hold ♥Q6 in a no trump contract. How many heart tricks are you going to lose? Heaven knows! If an opponent holds ♥AK10432 it could be as many as six. But in a suit contract (e.g. 4♠) you'll lose a maximum of 2 tricks, because after two rounds you'll be trumping hearts.
So how does it help?
It gives you guidance on how high to take the bidding. Let's assume an opening bid of 1♠, and further assume that the opener's partner has at least four spades.
Most 'ordinary' hands on which you'd open 1♠ will have 7 losers. Some stronger (and/or more unbalanced) hands will have just 6 or even 5, but if your partner opens one of a suit, you can assume for the moment that he has just 7 losers. (This might be a good point to look back over a few opening hands in your coursebook and count the losers. It's good practice and will also help to give you a 'feel' for what a 7loser hand looks like.)
Right at the start, we said that you add your losers together, subtract from 24 and that gives you the number of tricks you're going to make. That means (you can trust me if you can't get your mind around it. Honest) that to make game in spades or hearts (10 tricks) you should have no more than 14 losers between you.
Which can be very handy if your partner has just opened 1♠. With 4+card spade support and
 9 losers  raise to 2♠
 8 losers  raise to 3♠
 and with 7 losers, raise to 4♠.
Put yourself back in opener's seat. You open 1♠, and partner raises you to three, showing 8 losers. Do you go on to game? Count your losers: if you have the normal 7, pass. but if you're a bit stronger, with only 6 losers, sure  bid 4♠.
Here's an example in hearts.
North opens 1♥ with ♠K93 ♥AQ1054 ♦64 ♣AQ7. (= a 6loser hand)
South has ♠97 ♥K762 ♦K87 ♣J943, a pretty miserable 9loser hand. She raises to 2♥.
What does North do? Well, South has shown 9 losers, North has 6, so the LTC says there are only 9 tricks available: not enough for game. Pass.
Is this right? Well, I could construct two opposing hands such that North can make 10 tricks, but that would require the ♠A, ♦A and ♣K all to be in the 'right' hand: pretty poor odds. 8 or 9 tricks are far more likely. So yes  it's right.
Let's strengthen South's hand a little and give her ♠A7 ♥K762 ♦K87 ♣J943, so that she now has only eight losers. This time she raises to 3♥.
North does his sums and concludes that with 8+6 =14 losers they'll make 10 tricks, so bids game.
Again, this seems to work. The partnership will lose no more than 2 diamonds and 1 club, and will do better if the opponents' cards are well placed.
But what if South has something like this? ♠A7 ♥K7632 ♦J8752 ♣3? Pretty weak in high card points, but only 7 losers. This is where counting your losers really comes into its own: bid straight to 4♥. Not only will this make it difficult for the opponents to get into the bidding (and find their probable spade fit), but it's also a great contract, making at least 11 tricks.
Raising responder's suit
Of course, you don't always find your fit immediately. Take these two hands:
North: ♠J8 ♥KQ65 ♦72 ♣AQ976 South: ♠A5 ♥A1054 ♦1083 ♣K54
Let's say North opens 1♣ and South responds 1♥. They've found a fit in hearts. With a 'normal' 7loser opening hand, North would raise his partner to 2♥, but with only 6 losers, he shows his extra strength by going one level higher to 3♥. South now knows that with her 8 losers, she should go to game. (And if you don't get a diamond lead, you could well make 12 tricks.)
It's just a guide
A final word: as you've seen, this is a pretty rough and ready method of assessing your potential trick haul. It works a lot of the time, but it isn't gospel and it isn't a law. It's just a guide: one of several tools you can use to help evaluate your hand. You'll also count your points and take opposition bidding into account. There are other gadgets, too, including the grandsounding Law of Total Tricks, of which more later.
Meanwhile, get the habit of counting losers. It pays off a lot more often than it lets you down.
IMPORTANT
Don't use the LTC to downgrade a perfectly good hand.
I recently opened 1♥, partner raised me to 3♥ and, having an ordinary opening hand with 7 losers, I passed. Partner then proceeded to lay down a dummy with 4 hearts and 13 points. We had an easy game in hearts, as you'd expect. So why didn't he raise me to game?
'Well, I had 8 losers, partner ...'
I recommend the following, once you've found a fit:
Count your points and see what bid that suggests. Then count your losers and see what bid that suggests.
If they're not the same, bid the more optimistic of the two.
In other words, use the LTC as a way of upgrading your hand. OK, you may want to temper this with caution depending on vulnerability and the opposition's bidding, etc, but I find it works well for me. Try it!

