Here's a link to a rather classic Sound on Sound article on headphones mixing.
http://www.soundonsound.com/sos/jan07/articles/mixingheadphones.htm
Most of the important points are well covered and you should pay special attention to the "Simulating The Loudspeaker Experience With Crossfeed Plug-ins" box around 2/3 down. If you really can't have monitors around, crossfeed systems can help quite a bit. Mixes should always at least be listened to on speakers before stamping them as final, though.
IHTH
JC Boulay
Technical Director
Audio Z
Montreal, Canada
http://www.audioz.com
