If I consider the price good, I buy. And stop checking it. After the purchase, the price can increase, decrease or remains the same. In one trip I may win, in other I may loose... It's part of the game.
Personal example: I bought in January a ticket for New's Eve. The price was good (for me), based on previous trips for the same place, and I really want to be at this destination. If the price changes, it'll be at maximum a couple of hundred dollars - I can live with that. And I don't want to jeopardize my vacation worring if I could make a better deal, or pissed because I found a better price. Vacation by definition is a splurge - and I'm not going to really count beans doing a splurge...
I think the same way, I usually have a target price on a given trip, if the price of an airfare goes below that target price I buy the ticket, I don't care if it can get cheaper, a friend of mine used to say: "if you are satisfied with what you bought and how much you spent, its a good deal".
However since joining FT buying a ticket also means knowing how many miles I get from it, often here in Brazil airlines do fare sales where promo fares earn 0 miles while on a normal day you would pay almost the same for that ticket and still earn miles.