Save in front of it. Then talk to it. If it isn't shiny turn off the system. then turn it on again. Keep doing this until it's shiny. (Also works for starter pokemon in some games)
If you have an action replay you can easily get them. But first you must have the walk through stuff cheat. To get Darkrai you must go to Fullmoon Island. Once you get there start going right. Just keep going right and eventually you will get to another island. Darkrai is there. It's on level 40. For Shaymin, go to the big stone tablet thing on Route 224. Then start going up you will soon get to a extremely long island. At the end is Shaymin. It's only on lv 30.
Press A in front of the TV in the old chateau between 8:00pm and 4:00am. It does not always appear. If it doesn't save in front of it and try again after about 30 minutes. I got mine at about 9:35 or so. Mine was on lv 20 but it's not always on the same lv.
If you kill any legendary pokemon just beat the elite four and they will appear in their original places again.(Roamers will roam again.)(For the legendary birds you also have to talk to Professor Oak again.)