(If I can humbly interject a thought here: very often we do not pray the Jesus prayer not because we are unable but because we are corrupt wishing to think other thoughts and do other things).
St. Neilos advises: Always remember God and your nous will become heaven.
Archimandrite Hierotheos Vlachos
"A Night in the Desert of the Holy Mountain"
Birth of Theotokos Monastery, Levadia, ISBN: 960-7070-04-6