You just save and battle them and soft reset until you see a shiny... It took me a few hours to get shiny Lugia. I've tried getting my own shiny Giratina in my Pearl for days now and still nothing. So it's just luck and some determination and patience also helps. It's quite worth the effort though. ^_^
Edit: It doesn't increase you chance... Even if you soft reset 10,000 timesm you still have a 1/8192 chance of seeing a shiny. :P