First of all, let me say that guide books are a wonderful resource. Everyone buys them and some people follow them religiously. I don't mean to discourage the use of guide books, quite the opposite... often times they will let you know exactly where the crowds will be and (depending on the level of detail contained in the book) exactly when too. Some titles give recommendations on when to visit certain sites for the best sunset, best view, etc. This will help you become aware of when crowds are likely to congregate and how to time your shot list (you do have a shot list, don't you?) Also, look at the brochures of local tour companies. They usually print a schedule with times, places, etc. and if they're a successful company, you'll be able to count on crowds at these times.
Let's say you're at Angkor Wat in Cambodia and the Lonely Planet says that the temple Ta Phrom is best at sunrise, try visiting at sunset instead. Your photos will look completely different from what the masses are capturing and you have a good chance of having the place to yourself. Many historic sites are marketed as a "circuit" to help the flow of tour groups. The order and direction in which they are visited is often based on convenience and not on the best available lighting or time of day (great photos are not convenient). Scout your location the day before and then arrive the next day well before sunrise or linger on after sunset. Most tourists won't put the time in to get a great shot so make these your prime time office hours. The following examples were shot at the "wrong" time of day according to the guide books.
With that being said, often times there is a reason why guide books recommend visiting sites at a particular time of day... LIGHT! Sometimes it's just going to be impossible to avoid the crowds. One of the most beautiful (and most photographed) slot canyons in America, Antelope Canyon, is only accessible with a guide and only at certain times. There is a 'window' of about 15 to 20 minutes during the day when the sun is directly overhead and creates those amazing shafts of light in the haze of the canyon... and everyone else knows it too. You'll run into all sorts of people from pro photographers to groups of tourists with kids. In this case you'll have to resort to a different technique.
Try putting the camera on a tripod and opening up the shutter for a longer amount of time. By exposing for 10 or 15 seconds or longer, most people will simply blur out of your photograph as they walk into your shot and you'll never be able to notice that they were there. The use of a Neutral Density filter may help facilitate this if there is too much light to expose for longer periods. These filters effectively decrease the amount of light reaching the sensor in your camera allowing you to open the shutter for longer periods of time to get the right exposure (and they do wonders for moving water too, creating a soft blur instead of sharp ripples). Cokin makes very affordable resin filters that slide in front of your lens with the use of their filter holder. There are other companies that make glass filters too but these are much more fragile and expensive. The following images are examples where people have walked through my shot only to blur out in the final image. It's even hard for me to tell which ones had people in them at this point.
