If you want to make it look dirty, try to be logical about where you dirty up the paint instead of adding random splotches. There would be a trail of built up grime eminating from the cowling gunports, as well as a lot of oil and dirt particles building up over time as big streaks of dark grime coming from the exhaust ports. Finally there's the dirt and scuffing from the pilot walking on the wing where he enters and exits the plane, as mentioned. After that you have the more evenly spread out weathering effects like peeled paint and a bit of rust. Also keep in mind that the top surfaces get sunbleached and tend to look a little lighter and more washed out than the undersurfaces.
