How do I give a written notice to tenants?

[George Pan]

Hello,

I'd like to not renew the lease on my current tenants.
Based on my lease, if I don't want to renew I must give my tenants 30 days written notice prior to the end of the lease.
Now, how do I make sure they won't turn around and say "Hey, we never got that notice!".
I was thinking of sending it by certified mail.
How do you send written notices to your tenants?
How do you prove that you've sent something?

Thank you