You're doing it exactly right. Just make sure you're running around in the grass long enough, sometimes it takes longer than you would expect for something with a high encounter rate.
I have just finished the elite four and can't find Raikou
Have you fixed Celio's machine? IIRC, you have to do that before you can find Raikou.
Yes I done that but haven't done the Saphire/Ruby task yet.
If you weren't using the masterball, then I'd suggest you use Mr. Mime because he's not effected by sound based attacks. But otherwise, just keep going around, you'll find it...eventually.