The best way to make miniatures is to go with what's simple. Anything can be used. Cereal boxes, model parts, whatever.
The best thing I've found, however, is model train parts. These are always in scale and very detailed. the downside is that they are expensive and not always hollow.(For blowing them up, if you want to.) They even have little people you can use.
If you're making a futuristic city, go straight for the metallic paint. Also use lots of domes and pyramids for buildings.
If you wish, you can buy those electric models, or make your own models and light them up from the inside.
You can construct your prop buildings with drywall, or mix vermiculite into wet plaster (to make them crumble easier) and create them from scratch.
You can make small buildings with balsa wood, corrugated cardboard, and styro or polyfoam; paint your little models with runny plaster. Once dry, the plaster will flake off when you blow up the building, or stomp on it, or whatever, and it'll look like large hunks of concrete falling if you put it in major slo-mo.
