I organize some by date, some by topic (if the topic is pretty important, like my wedding
) I go to the huge Sema show in vegas every year and take a bajillion pictures there, usually that gets its own folder, sometime broken down into makes (dodge, bmw, etc). or trucks/cars/ mutated ill-tempered sea bass with frickin laser beams - you can see most anything in vegas
If it's a big trip I'll give them it's own folder