Make sure you have plenty of repels (max,super,regular as it doesnt matter) then go to Slateport city. Make sure you have a Pokemon that is level 25-39 in front, it can't be above level 39 (Latios/Latias is 40 so if you are higher than that you will repel it) Use a repel and run around in the grass that is just below the cycling road building. Run for about 20-30 steps then go inside the building and out to run in grass again. Keep doing this until you find him. When you do see it USE THE MASTER BALL because it WILL run away when it's his turn (even if it's asleep, frozen, etc.) trust me I caught the regi‘s and rayqauza with ultra balls fairly easily. If you used the master ball already then you‘ll have to whittle down it's health over time and then hope that you catch it with your first ultra ball.
Added by: Guest May 1st 2011, ID#13471