Vajrasattva Mantra

The definition of negative karma: any action that results in suffering, usually an action motivated by ignorance, attachment or aversion.

The actual practice comes with visualization of the Four Opponent Powers:

http://www.lamayeshe.com/index.php?sect=article&id=394 (with explanation added)
http://www.fpmt-osel.org/meditate/vjrstva.htm (A shorter 'summed' up content)

The Power of Regret
The Power of Dependence
The Power of the Remedy: Mantra Recitation
The Power of the Promise

...but its good to have the mantra in our space even if we are just reciting the mantra without the visualization.

Long Mantra
om vajrasattva samaya
manupalaya, vajrasattva denopa titha,
dido me bhava, suto kayo me bhava, supo kayo me bhava,
anurakto me bhava, sarva siddhi me prayatsa,
sarva karma su tsame, tsittam shriyam kuru hum,
ha ha ha ha ho, bhagavan
sarva tathagata, vajra mame muntsa,
vajra bhava maha samaya sattva ah hum phet

Short Mantra
om vajrasattva hum