What is Nitya karma?  what are the principle means by which it is
accumulated? 

Does it offset or neutralize bad karma from the past?

There are four kinds of karma:

1. Samskipta-past, over and done with, a present memory.
2. Bartman-present, what is happening right now.
3. Bhavishya-future, the actions begun in the past, the fruits of which will be realized in the future.
4. Nitya-Eternal.

The Past is a present memory, the Future is a present projection. The
only reality is the Present: Be Here Now!

There are eight forms of behavior prescribed for all spiritual seekers:

• Vaishnavachara — find your inspiration
• Vedikachara — learn about that which you love
• Shaivachara — practice that which we learn about
• Vamachara (beloved behavior) — perform every action in life efficiently
• Dakshinachara (preferred path) — reduce your necessity to perform
  worldly actions
• Siddhantachara — make your behavior correspond to the seven activities
  spoken of in the scriptures:
  • Puja (Worship)
  • Path (Recitation)
  • Homa (Sacred Fire Ceremony)
  • Sangeet (Singing)
  • Nrit (Dancing)
  • Pravachan (an explanation of the meanings and applications, how to
    integrate these various activities into our lives)
  • Arpana (offering)
• Yogachara means “behavior of union”.
• Kulachara (behavior of excellence) — whether sitting still in
  meditation or actively pursuing some objective, the attitude toward
  life remains the same in every circumstance.
• Siddhantachara is another name for Karma Yoga, the actions by which we
  come into union. Any action performed in that Union is Nitya Karma.

As we fill our Present with Eternal Karma, there is less and less
space to consider the Past or the Future. When All we have left is
Nitya Karma, then we are totally in the here and now, totally in the
Present.