Self-defense is defined as the legal use of force when a person believes it is absolutely necessary to prevent injury or death to his person. Circumstances vary from case to case, but there are some general guidelines you can follow if you wish to plead self-defense to an assault charge.
Hire an attorney who specializes in assault cases. Well-trained lawyers are your best friends when criminal charges are pressed. A good attorney will often know how best to built a case for self-defense, and can advise you on what to do.
Explain why assault was the only option. A self-defense argument usually succeeds only if your actions came as a last resort. Be prepared to explain why other means of resolving the situation (such as retreating from the situation or calling the police) were not feasible in your case, and that you risked injury or death if you hadn't acted immediately.
Point out that a felony was being committed, or that your assailant struck first. You are legally allowed to defend yourself if the other party is committing, or is about to commit, a crime. You may also legally defend yourself if you were attacked before attacking someone yourself. If you can demonstrate either of these two conditions, you may be able to plead self-defense successfully.
Show that the force you used was proportionate to the force with which you were threatened. The law weighs proportion very carefully in these cases. Be prepared to show that you did not escalate the situation, and that you fought back using the same means and force with which you were attacked.
Demonstrate that you stopped when your assailant no longer posed a threat. Your ability to plead self-defense successfully may hinge on how much damage you inflicted after your assailant was clearly incapacitated. You should note when you stopped attacking, and be ready to explain why your assailant was still a danger up until that moment.
Eyewitness accounts can be very important in assault charges. If you know of someone who was on the scene and can support your version of the events, urge your lawyer to get in contact with her. Pleading self-defense to an assault charge is often easier than pleading self-defense to a murder charge, because you did not kill the person (and thus demonstrated an amount of restraint). Most self-defense laws involve a "duty to retreat" rule, meaning that you're expected to get away from the situation if you are at all able to do so. However, the duty to retreat does not usually apply if you are in your own home (the so-called "castle exception"), making self-defense much more justifiable in those situations.