In order to prevent counterfeiting, mints have always been trying to be one step ahead of counterfeiters.

And thus, they think of ingenious ways to do such.

Some of the easiest to the hardest that I can think of are: (they might not in any particular order)

1. Alloy mixture
2. Mass of metal per coin
3. Size restriction (i.e. +- 0.01mm on height and diameter of coin)
4. Edge
5. Planchet shape (hey, counterfeiters have been perfecting on how to make perfect circles and when it comes to unusual planchet shapes, it can cause trouble!)
6. Bi-metal / Tri-metal
7. Holograms

Are there any more securities besides these? I am sure that I might have missed some.

Have you no modern Russian 10 roubles? How could you forget latent images?

Here is a page from the Japan Mint.

Yes... I guess that is what I mean by "hologram"

Ironically I have both of them in my collection