Pokémon Diamond shiny Pokémon?

Actually, the best ways to get shinies depend on what you're looking for.

For example, if you want a shiny starter Pokemon, you can either soft reset (L+R+Select+Start) at the beginning of the story where you first pick your partner (save first), or you can breed for it (the first is the easiest unless you're lucky).

The only way to get a shiny legendary Pokemon is to save in front of the Pokemon before interacting with it and soft reset until you get the Pokemon as a shiny.

Another way is to just run around and hope for the best, but you have a 1/8192 chance of getting a shiny by just roaming around aimlessly, so that's the worst possible scenario. Breeding for that Pokemon is pretty much the same deal too.

The easiest way to get a shiny is to beat the game, get the national dex and the PokeRadar and chain the Pokemon you want up to 40, then reset until you see a shiny patch (trust me, you'll know what that looks like when you see it). That's about it. :)