Over the years I've discovered some ways to avoid or reduce inattention:
1. Don't do anything else while chanting Japa
2. Pronounce each word very clearly...even if it means slowing down your chanting
3. Listen very carefully to the sound coming out your mouth...some devotees stand in a corner, others hold a cupped hand to their ear to hear more clearly
4. Read the Mantra whilst you chant (in Temples this was a common addition to the Temple room...the Maha Mantra on the wall)...I personally found this a very effective way to keep the mind from wandering.