I know this applies to R/S/E cause I have tried it, but not sure about other Gens and I know this is different for Gen IV
In R/S/E, can you find the roaming Latias or Latios? You see it roaming around but you can't battle it or find it in same area your in?
Well, what I have done is this:
Put your highest level pokemon as first in your party (must be over level 50 anyway for this to work and preferabilly have one that knows quick sleep enducing move or paralysis move if you don't want r have not got a masterball to spare.) The go to the clump of grass I think just past Slaterport (near the cycle route) and search around, if you see in your pokedex that the pokemon your after is not there, go into the cycle route building (the one that you have to go in with your bike to access the cycle road), then come out.
You'll now see the location of the pokemon has changed, it changes everytime you change route or place. So the easiest way of catching this pokemon would be to keep going in and out of the cycle route building until you find the pokemon in the area your in, just search the grass nearby (in the same route) and you'll find it! But, best thing to do is before going into grass, use repels to maximise your chances of finding it quickly
Now I have not been able to catch it yet, just find it. Don't try and switch out pokemon when battling it, unless you have used a blocking move such as mean look, it will flee as many of you will know.
Thats what I have done for R/S/E, if it works on other Gens, I have not tried it yet.