Multiple locations: One of the main reasons tropical vacations often seem like they disappear so quickly is because you are simply relaxing on the beach the whole week, and this leaves you with no concrete memories you can point back to as the week winds down. One way to counter this is by staying at multiple locations during your vacation; if you are visiting an island, consider splitting the week between two different sides, or if you are visiting a country, divide your time between two or three different beaches.
Participate: You can also create concrete memories by making sure you participate in the activities that are offered at the the resort you are staying at, or in the local village or town. Rather than simply sitting on the beach all day doing nothing, play some games, see some sights, and do what you can to create memories that will outlast your trip!
Meet people: And of course, one of the best ways of all to extend the time of your tropical vacation is to get to know other people who are staying at the same place where you are staying; as you strike up friendships with others around you, you will be able to share the experience with more people, and in this way you will stretch out your vacation time!