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And some luck as well.

Plus, you have to be a bit bigger then he, having a lot of armies and cash is said to help as well, also a good stability, good monarch diplo skills and some more factors.

Care to describe your problem a bit more in details?
 
What you MUST have, is: royal marriage, relations of 190+, and have them in the alliance you lead. The longer this remains true the greater your chances. Also the less stable they are, the less capable of defending their own borders, and the more under threat from a nearby enemy, the more likely they will accept. Just after ending a war in which they lost half their troops is a good time to ask.