How to prepare for an outdoor adventure

I once went to the mountains with friends without any proper preparation - no first aid kit, no map, not even a flashlight. We thought we were just going to walk around for a couple of hours. But we got lost in the forest, and had to wait until the dawn showed us the way. Since then I take any hike seriously: it's better to spend half an hour packing than to spend the night on the rocks. How do you prepare for such outings? Maybe you have some of your own tricks or a list of things?