I heard that it was possible to purchase another plane ticket very cheaply if you were flying to the same destination within a certain period of time. Is it true? How do you do this?
There used to be some tricks in this area if the airline sells significantly cheaper return tickets if the stay included a weekend. Some airlines did this on the assumption that the more price-sensitive leisure travel was more likely to include weekend stays than the less price-sensitive business travel.
Suppose you live in city A and need to be in city B on Tuesday and Wednesday of each of two consecutive weeks. The natural choice is two A to B return tickets, each outbound on Monday and inbound on Thursday of the same week. Neither includes a weekend:
Ticket 1 Outbound Ticket 1 Inbound Weekend Ticket 2 Outbound Ticket 2 Inbound
Now consider making Ticket 1 an A to B return, outbound on Monday of week 1, inbound on Thursday of week 2. Ticket 2 is a B to A return, outbound on Thursday of week 1, inbound on Monday of week 2. Each ticket has a weekend between the outbound and inbound flights:
Ticket 1 Outbound Ticket 2 Outbound Weekend Ticket 2 Inbound Ticket 1 Inbound
I think giving preference to weekend stays has become less common, so this is an obsolete technique.