Muggles' Guide to Harry Potter/Magic/Love Potion

Overview
A Love Potion is a potion that makes the drinker "fall in love". Generally these potions are somehow "keyed" to an individual; the person who imbibes the potion will fall in love with a specific individual, usually the one who prepared (or at least performed the final steps in preparing) the potion.

Extended Description
In Harry Potter and the Half-Blood Prince, Professor Slughorn states that you cannot truly create actual love, the only thing the potion brings is obsession or infatuation. Apparently, love potions are fairly common; Fred and George, for instance, are apparently doing a pretty brisk business selling them to the girls at Hogwarts.

Analysis
Love potions do not figure particularly prominently in the story until Harry Potter and the Half-Blood Prince. Up until Harry's fourth year, Harry and his classmates are too young to be overly concerned about such things. The sole mention comes from outside: Rita Skeeter, in writing about Hermione in Witch Weekly during Harry's fourth year, quotes Pansy Parkinson as suggesting that Hermione may be using love potion to keep both Harry and Viktor Krum on a string.

The conflict between the Champions in the fourth book, and the Ministry campaign against Harry in his fifth year, make him not a particularly desirable "catch", so love potions are not something that anyone in the story has need of. By the beginning of Harry's sixth year, though, with the Ministry electing to blow up Harry's significance and make him "The Chosen One", several girls, most noticeably Romilda Vane, are scheming and conniving to get hooked up with him; and love potions end up being a big part of that. The first solid mention of a love potion is the Amortentia potion, shown by Professor Slughorn by way of example in his first Potions class, and identified by Hermione.

In the days preceding Professor Slughorn's Christmas party, Hermione hears several girls scheming about ways to slip Harry a love potion, and warns him. As a result of this, he turns down an offer from Romilda Vane to share a Gillywater, but does accept a box of chocolate cauldrons with Firewhisky centres, which he carefully sets aside, expecting them to be spiked with more than alcohol. Ron discovers them on his birthday, some months later, and assumes that they are a birthday present for him; he, of course, promptly becomes infatuated with Romilda, completely forgetting about his current entanglement with Lavender. Professor Slughorn is able to cure him of this infatuation in short order.

One other use of Love Potion mentioned in this same book is the use of it on Tom Riddle by Voldemort's mother. After using the potion for a while, she incorrectly believes that Riddle Sr. is truly in love with her, and discontinues the potion; he, feeling betrayed, abandons the now-pregnant Merope and returns to his family home.

Questions

 * 1) We never see a Love Potion brewed in the series. What would be some possible ingredients?

Greater Picture
The two times we see Love Potions being used, they are very specific in their effects: the potion used to spike the Chocolate Cauldrons, as mentioned, gave Ron an instant infatuation with Romilda Vane, and the potion we believe Merope Gaunt used on Tom Riddle Sr. similarly had Merope as the object of desire. This would seem to be incompatible with the Twins selling love potions by owl post; how could the Twins know who the love potion was supposed to be causing infatuation for? This begins to make a little more sense when we think back on the brewing of the Polyjuice Potion: the potion was not complete until some hair (or presumably fingernails, toenails, or other "shed" matter) was added to it. What the Twins presumably shipped would be incomplete; the recipient would have to then finish it by adding something to identify the target of affection. So for instance Romilda could have received potion from the Weasleys, added a bit of her own hair to it to aim it at herself, and then used it to spike the Gillywater that she offered Harry, and the Chocolate Cauldrons that she gave him.

Alternately, when ordering a love potion, the order form may require you to include a lock of hair. But that could cause serious headaches for the Twins if hairs got mixed up. We can imagine Romilda's fury if the potion she had ordered made Harry infatuated with, say, Lavender Brown... We won't speculate on the sort of practical jokes that could be caused by creating a love potion that would cause two very inappropriate people to become infatuated with each other, except to suggest that the only reason the Twins aren't seen to be doing this is likely because this is a book series meant for children.

The Amortentia potion that we see in Harry Potter and the Half-Blood Prince is presumably unfinished, needing only a lock of hair, or equivalent, to make it effective.