Remote logging with scp without entering password.

On the source server,
ssh-keygen -t rsa
copy the id_rsa.pubto remote server's .ssh/authorized_keys

Possible reasons why the scp will not work are  :
On the remote server check

Authorized_keys needs perms of 600 (chmod 600 authorized_keys)
The .ssh directory should be 700
Your home directory should be at most 744