Long answer:
One: Learn just a little something about the laws regarding trespass in our jurisdiction. In many states, you may ask a trespasser to leave, and if he does not comply, you may only call the police. In some states, the police may only issue a citation--rather like a traffic ticket. In no state may you use deadly force for trespass alone or to enforce a request to leave.
Two: Find out whether you have a duty to retreat in your state. Don't rely on the criminal code--it may have been defined in legal precedent.
Three: Figure out what evidence you might be able to present in your defense should things go south after you have elected to not retreat, if retreat is not required. With any luck at all, you will not have advanced at all.
Short answer: Make your self scarce before anything gets uncomfortable. Keep your health, your money, your clean record, and your right to own a gun.