I don't know what people mean when they say penpals, most of the time I think they mean 'date'... Anyways, this is my mini guide to the truth about cats and dogs....
☆pattern your speach in english around sparse isolated phrases, life, love travel, fun, friends, etc... you can write more, but as someone said, don't write a book about your life.
☆many people who check that they can speak english often can not speak it fully, just as people who check that they can speak japanese cant either... know that the person who you are speaking to may not completely know the language--- use a mixture of japanese and english for best effect.
☆talk about things that you like, not things you don't like (obviously)...
☆show an honest interest in the culture of the person whom your trying to meet
☆dont talk about how much you just loooove anime or momusu just yet...
☆talk about where you have been in the world and where you would like to go
☆be yourself, natch. have a personality--- (your milleage may vary)...
use UNICODE! ☆☆☆

(or not...)
edit: Oh yeah, I forgot to mention this--- I know a lot of people who are in Japan that either for some reason or another are looking for 'penpals' who happen to ALSO be in Japan... it's difficult to keep in touch with people when they are half way around the world and this is a big factor into the depth and length people will put into a friendship and communication.