Good stretching techniques?

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I've always had a problem with hyper-extending. More specifically, my knees. I'm just curious if any of you know of any good YouTube stretching tutorials or something? haha

And is it recommended to do it after exercising too? I heard so somewhere that it is. I'm sure my trainer will help me more when I stat the gym next week.

Any tips and/or advice would be fantastic. THANKS!