What you got to do is make sure that none of the strings are touching the the yoyo. And pull it tight.
If you have a yoyo with a KK or a CT it really helps keep the string from touching the yoyo.
I hope this helps.
dont pull as hard and move hands away from walls of the yoyo 8)
Also, a yoyo without a highwall really helps too. Along with using the thinnest string possible.
This tut helped me out alot watch the part at 1:30on he explains the movement needed to keep the strings from touching