Some people join in many penpal sites. Tell your hobbies and interests to your penpal. Even if you don't, your penpal will get to know more about you. If you write your interests are things like music or movies or books, tell them what kind of books you like, who is your favorite movie, books etc. Yours' or 'Sincerely' is OK too.
Women sometimes write 'Love' to their female penpals, even in their first mails.
But if you're a male and write to a female penpal for friendship, never use 'Love'. It is scary. You shouldn't write something that forces your new pal to write you, such as 'Whatever happens, do write back'. It makes your new pal feel bound. Your penpal will be sure to answer if you send a nice mail.