"I'm going to have to ask you to leave" is the legally proper way to ask someone to leave a residence/business/property. Saying this avoids issuing someone a direct order, which when issued by an authority figure such as a security guard, could possibly lead to legal complications. For example, "get out" is an order, and in theory, COULD be illegal if the party you are requesting to leave is not commiting an illegal act, or you are not preventing an imminent breach of safety.
After you have asked nicely, and the person remains, they are trespassing, thus entitling you to use whatever language you like to remove the person, because you are trying to get them to leave to stop them from illegally trespassing.
Added: it is a legal grey area to order someone to do something, because by law, if you restrict someone's freedom of movement, you have technically arrested them. If they are allowed to be in the place they are in, then you cannot /should not order them to leave until they refuse to do so. This applies to anything that you reasonably have a right to protect, such as your home, your business, or other human life.
This is a huge grey area, and all this is based on my training, which may be played up so my company can avoid lawsuits. You are allowed to use only the minimum amount of force required to correct a situation. If someone is in your home, after requesting they leave, you may physically remove them by pushing them in the proper direction. If they push back and resist, you may then step things up a notch. It is much easier to just call the police. All this is null and void, however, if you believe your life is about to end. In that case, whatever force necessary is allowed, within reason. Fearing for your life is not enough. A person cannot just be in posession of a weapon, it must be brandished in a way that causes you to believe that you are about to die.
Thanks for the questions, fellow noders, you helped me clarify this properly, which I seem to have a hard time doing the first time around.