Maybe it's air fares to Europe, the cost of renting a house boat for two weeks next summer, all known symptoms of hoof and mouth, or the latest rumors about the price of oil. Your site has a purpose for being. If all is working well, your visitor arrived in accord with that purpose looking for information you can provide. Do so immediately. To hit a visitor with a sales pitch is dumb.
All surfers know where the Back button lives.
And they use it frequently. That is, think of what you most want your visitor to do relative to what your visitor most wants. It doesn't take a lot of brain power to realize you and your visitor do not ultimately want the same thing. Taking this a step further, it should be obvious that what you want is meaningless to your visitor. Thus you are whipping a dead horse if you do anything other than seek to provide your visitor with exactly what is wanted.