I have come to the conclusion that you are pretty much never supposed to make a bracha achrona. I don't assert that this conclusion is correct. In fact, it is so crazy that I plan on asking my posek about it ASAP. But in the meantime, it makes perfect sense to me, and here is my logic. None of what follows should be treated as halacha, because it probably isn't, even if it seems to me to be correct.
First, and most importantly, we have the principle of "safek brachot lehakel". Which is to say, when you are uncertain whether to make a bracha, you don't make it. Quite possibly, this rule does not apply when the bracha is d'oraita (birkot hatorah and sometimes birkat hamazon), but these situations normally don't concern us here. Furthermore, "safek brachot lehakel" is so strong that it seems to override other fundamental rules about brachot. For example, what if you want to eat something but aren't sure if you made a bracha on it? You have to decide between the principles of "safek brachot lehakel" and "assur lo leadam sheyihaneh min haolam belo bracha". In this case, most authorities hold that it's preferable to eat without a bracha than to potentially make an unnecessary bracha.
Second, a bracha achrona should only be made after eating a k'zayit of food within a time period known as k'dei achilat pras. The latter is generally held to be 3 minutes (others say 4 minutes). There is much disagreement as to how much a kzayit is, but the opinions seem to be from 15 to 50 cubic centimeters. According to the principle of "safek brachot lehakel", I think we should only make a bracha achrona when these two measurements are satisfied according to all possible opinions. In practice, that means you only make a bracha after eating 50 cm3 of a given food within 3 minutes.
Some of you might not be aware how much 50 cm3 is. According to a quick Google calculation, it equals a cube nearly an inch and a half on a side. This is a lot. Try eating this much of a given food while watching the clock. Eating nonstop, I think you will find that it takes a large part of those 3 minutes to eat a single k'zayit. In normal social situations, or when eating candy or other such foods, your pace will undoubtedly be slower. Furthermore, it is difficult to measure exact volumes of food and (sometimes) time, and due to "safek brachot lehakel", you only make the bracha if you're entirely sure the conditions are satisfied, which rules out substantial boundary regions as well.
In some cases, there's even less chance to make a bracha achrona. I believe you do not make a bracha on the food unless you ate an entire k'zayit of the "ikkar" of a food combination, not including the amount of "tafel". Thus, if you made the mezonot bracha over pie, you would never ever say "al hamichya" afterwards, because it's not humanly possible to eat 50 cm3 of pie crust and the accompanying filling in 3 minutes. This would also apply, to a greater or lesser extent, to all other foods which are combinations of mezonot and other stuff, like hamantashen, lasagna, and so on.
Thus, I've come to the conclusion that there are very few situations when you actually make a bracha achrona. It's just very unusual that one eats 50 cm3 (or sometimes, a much larger quantity) of food within 3 minutes. Of course, this opinion conflicts with the whole world seems to do, and everyone who's heard my explanation thinks I'm crazy (without providing logic for or against me). In addition, on a behavioral level, it seems to conflict with the statement I remember from R' Bodner's Kzayis book, that a normal person eats about 6 kzaitim (sp?) per 3 minutes. These discrepancies could be resolved by pointing out a fundamental logical flaw of mine, which I haven't found, or else by defining the k'zayit to be less than the value of 50 cm3 I made use of.
While my conclusions may in fact turn out to be wrong, I do think that my thought process was reasonable. Everyone agonizes over whether they have eaten the correct amount of matzah on the night of Pesach, but the other 364 days of the year we seem to ignore the laws of k'zayit. Why the difference in attention? It's not so hard to get halachot such as this correct. I see no reason not to think critically and clarify them to the fullest extent possible.