Sometimes a strong interest in a particular period or place is enough to send a young traveller back through time. The setting is the Rocks district of Sydney, a warren of steps and narrow streets and old buildings that has changed little over the past century. In my own Fool's Gold, it is a gold nugget, lost by a young miner and found by a modern woman, that leads to the translation in time. A photograph is a very useful catalyst since it preserves a moment in time. A drug or medication is another possibility for triggering temporal displacement.
Psychic ability is a useful technique, because it hovers on the borders of sci-fi and reality.
Another useful technique is the use of some kind of device. In Jenny Pausacker's Fast Forward, young Kieran has an unusual remote-control. A clock or a calender would also do quite well. Another quite common method is a device or experiment from the future. This sounds rather hokey, but it has an underlying logic.