I have (16th July 2007) just added a new page on fruit thinning but decided to leave this text up as well for a bit of compare and contrast. fruit thinning page with photos
Some apple trees (for example Winter King, Miller's Seedling, Spartan) routinely set too much fruit. How much is too much? When there are more apples than the tree can carry to a decent size. Winter King (aka Winston) usually sets about 5 apples to a bunch, If they are all left, you will get 5 apples a bunch all the size of golf balls. This is no use to anyone, so they must be thinned, the operation is best done about a week after blossom has fallen, but can be done May through June. Use scissors or thin bladed topiary secateurs to remove the centre fruit (also known as the king fruit, this often has a short or deformed stalk and is undesireable) and 2 others to leave 2 fruits to the cluster. This can be done with a single snip using a thin sharp tool. If the tree is carrying a heavy crop, it might be wiser to thin most bunches to a single fruit-again, BALANCE, eye the tree over and make a judgement, your experience over a few years will tell you what is right. In a drought year you might need to thin harder than normal unless you can irrigate.
Another thing about fruit thinning is to do with formative pruning and the shape of the tree. If you have a main branch on a young tree, where the branch has not yet become really solid, apples at the end of the branch will pull it down to a lower position, in which it will set. If this results in a high angled branch being pulled down to the horizontal, great. If however a horizontal branch is pulled down to the floor, not so great. This may be acceptable depending of your view of the aesthetics of the tree, but if you are trying to mow underneath the tree it is a bad thing to have branches trailing on the ground. I therefore remove apples from the end of branches of new growing laterals in order to allow the branch to set at a higher angle. This seems wasteful, but it is not worth gaining a couple more apples this year at the expense of having to remove a whole branch in next winter's pruning because the branch has set in an unacceptably low position. Of course if you have time and energy you can 'maypole' i.e. put in a tall stake and support laden branches with strings. That way you get the good branch angles and also the current year's fruit.
A final word on thinning apples. If this job is neglected when it needs to be done, the result will be more apples, but smaller. I have learned from marketing that it is far better to have 1kilo of good apples than 3 kilos of bad ones (very small apples don't sell well, although we do a roaring trade in 20p bags of 4 or 5 baby apples for children). On top of this, since it costs the tree a lot in nutrients to produce all the pips (pips are protein-dense unlike the flesh which is mainly sugar and water), 400 apples weighing 20 kilos will wear out the tree much more than 150 apples weighing the same 20 kilos. The unthinned tree is therefore more likely to develop biennial fruiting (loads of fruit one year, none the next) which is a bad thing. We have discovered that fruit thinning is essential for many of our apples and few if any of the books I have read stress this enough. I suspect that some of the old varieties, like Winter King (Winston) for example which is a REALLY good apple (great flavour, very crisp, long keeper, extremely reliable, good colour), are not grown any more because of the need to manually thin the fruit every year. This is labour intensive so the industry doesn't want to know. Its quite a pleasant job of you don't have too many trees to do, but don't cut yourself. AS you work, the right hand is snipping away whicle hte left hand often holds the bunch of fruits to place them correctly. I once sliced the tip of my left ring finger open down to the bone, it still doesn't feel right 2 years later.