mitsu

Fly to any town.

Leave the Pokecenter and leave the town to the nearest route (this works with surf as well) and hunt around in the nearest patch of grass or water for a while (give it five or ten minutes, maybe more) if you don't encounter Latias, go back to the town (walk there) and then walk back out to the same place you searched and search again for the same amount of time.

Keep doing this until you encounter her.

It is important that you always go back to the same route, the reason being that Latias roams the same route she is in until you enter a new area, then she randomly moves to a different route on the map. If you keep leaving and going back to the same one this way, she will eventually come to the route you are in and thus you will probably encounter her if you give it enough time.

This may sound tedious, but it is really the most sure way of encountering her.

This also applies if you have already encountered her but failed to catch her without knocking her out.

If you knock Latias out, she will not be able to be encountered again. (this is the perfect time to use your masterball) Sobe careful if you don't have a Masterball. If you encounter her and fail to catch her or knock her out, she will flee the battle. If you attacked her, and drop her HP, the next time you encounter her, her HP will be at the same level as when she fled. This way is a pretty good way to catch her without the Masterball. Lower her HP and keep hunting her in the way I have described... Save your game after every encounter with her, but if you accidentally knock her out, reset your game without saving and you will be able to re encounter her.

Also, you have to have beaten the elite four and the champion at least once to be able to encounter Latias.