"Om Namo Bhagwate Vasudevaya"- probably the best Krishna mantra, that any devotee of the 'Natkhat gopal' can recite on Krishna Janmashtami or Krishna Jayanti. In case you want to plan to celebrate Lord Krishna Jayanti at home, you can do the above mantra, otherwise, you can also visit Krishna or Vishnu temples or spiritual organizations, that conduct various programs like discourses, dramas, processions etc on this day.
In 2012, the date of Janmashtami in North India is August 10 and in other parts it is on August 9. In Tamil Nadu, it is observed on September 8, 2012. In Kerala, Sri Krishna Janmashtami is on September 8, 2012.
At home, the Pooja involves, offering flowers and performing Aarti. You can even organize a Srimad Bhagavad Geeta discourse at your home, or can do it along with friends and relatives. Some devotees also, observe fast and spend time in temples or listening to discourses.
The main worship in temples takes place at midnight, when it is believed, that Lord Krishna was born. It must be remembered here that Lord Krishna is happy with anything that is offered to him in true devotion. The simplest of Prasad preparation can be, using mishri and tulsi leaves. According to our scriptures, Lord Krishna was even happy with sweat drenched beaten rice offered by Kuchela or Sudama. He was happy with the last leaf left in Draupadi’s Akaheyapatra. He was content with the gruel in Vidura’s (Dhritarashtra’s minister in Mahabharata) home.
Just remember the Lord truly and honestly, and keep doing 'Om Namo Bhagvate Vasudevay' for as much time as you can. You can also recite "Om Krishnaye Namah" from 108 to 1008 times. 
If you want to observe fast, then remember these three types of fasts are generally observed on Janmashtami:

Types Of Fast

  • Nirjal Fast

This fast basically means, starvation. Devotees would not consume even a drop of water, all through the day of Janmashtami. The fast is broken only in the midnight, when aarti is performed and prasad are offered to the deity. 

  • Phalahar Fast

Phalahar vrat is another type of fast observed on Krishna Janmashtami. This is somewhat lenient than the nirjal fast, because the person can consume milk and fruits during phalahar vrat. However, they refrain from cereals and salt. The fast is broken in the midnight, after Janmashtami Puja. 

  • Food For Janmashtami Fast

Since Lord Krishna is particularly fond of milk, a variety of dishes using milk and its products are made during the fast. Sweets like Peda, Shrikhand, Kalakand, Vella Seedai, Neyyappam are prepared by the devotees, to offer to Lord Krishna. Sweet dishes including desserts (preferably kheer) are made at the time, when people observe fast. However, they would not consume the sweet dishes or any other food unless and until they have been offered to Lord Krishna. Fast is broken at midnight.