Weathering Question

I know this is a real detailed area of modeling and this is probably an overly-broad question, but what are some general weathering tips? I've seen people talk about using pastels, but for what, and how? What about jet exhaust? I understand about chipped paint, but what about oil and gunport streaks? Anyone have any suggestions, or maybe a link to a tutorial on this subject? I've got this Tamiya F-4 that's all finished (minus the canopy which got lost somehow), so I figure I'd like to practice some weathering on this model but don't know where to start.