Mostly stuff I picked up from my father:
- Pencil crayons can be used for edge highlighting.
- No file? Fine grit sandpaper glued to the side of a popsicle stick can work as a reliable substitute in a pinch. The same also applies to sanding blocks.
- Turpentine is a good substitute for brown washes, especially if you're trying to invoke the appearance and texture of grime at scale. Mind the smell while it's still wet though.
- Windshield washer fluid and iso-propyl alcohol are both suitable cleaning agents for brushes and airbrush parts alike, and can in a pinch be used to strip paint off of models. However, iso-propyl alcohol can be exceptionally harsh on gaskets unlike washer fluid.
- Cotton balls are good masking filler for exposed cockpits when painting.
- To facilitate building a model, never skip out on dry-fitting the pieces together. If you must, lightly airbrush along the seams where the parts meet during dry-fitting, so as to be able to better identify where adjustments (typically via sanding) need to be made before you glue the parts together.