Summarize the following:
If the person won't even admit that they did something wrong, they're probably not going to apologize. Accusing them outright will often cause them to become defensive, but you may be able to get them to confess by asking them leading questions. At the very least, you might catch them in a lie that you can prove is untrue.  For instance, if you found out that your boyfriend was out with another woman, you might start by asking him something like, "Why didn't you answer the phone when I called you earlier?" If he says he was at work, you could say, “No you weren't, because I called there too.” If you know your teenager took cash out of your wallet, you could say, “Where did you get the money to go to the movies last night?” Be careful of accusing someone of something they may not have done. If they get blamed when they haven't done anything, they might start to feel like they might as well do whatever they're accused of. Playing the victim is an integral part of a guilt trip. Make it seem like this is a pattern of behavior you've come to expect, and certainly not one that you deserve. It's most effective if you bring up things that are similar to whatever you're trying to get the person to apologize for, but it can be anything that upset you.  For example, if you're trying to get your spouse to apologize for being irritable, you might say, “It's not the first time you've snapped at me. Remember the other day when you said...?" Only do this when you absolutely must, since it opens the door for the other person to bring up things they feel you've done to hurt them as well. Tell the person that what they did makes you question the way they feel about you. In some cases, this might very well be true, especially if what you're upset about is something very serious. Either way, make sure the person you're guilt-tripping feels like they need to prove that they still care about you.  Say something like, "How can you say you love me and then lie to me?" You could also say "It really hurt my feelings that you forgot my birthday. Don't I matter to you?" Put their actions into stark contrast by bringing up some of the good things you've done for them. The more recent or grand the gesture, the better, but anything will work. It doesn't even have to have anything to do with the subject at hand; just so long as it shows what a good person you are.  If someone takes something from you without permission, you could say, "After everything I've given you, how could you steal from me?” You might also say something like, "I guess it didn't mean anything to you that I brought you flowers the other day, since you want to pick a fight with me today." You could also say something like, “I've cooked dinner for you every night for 5 years, but you can't remember to pick up a carton of milk on the way home?” Overdoing this can actually make the person less grateful over time, since whenever you do something nice for them, they'll be wondering how long it will be before you throw it back in their face. When you're giving someone a guilt trip, they'll often try to do the same thing to you in return. Even if you've done something wrong, don't acknowledge it. Instead, turn everything back on what the other person did.  For instance, if you want your boyfriend to apologize for texting another girl, he might try to make you feel guilty for going through his phone. In this case, you could say something like, "Well, it turns out I had a reason to be suspicious, didn't I?" Even if you lose your temper, you can say something along the lines of, "I wouldn't yell if you didn't make me feel like this!" Refusing to admit when you're wrong might be an effective technique for a guilt trip, but it's not a good approach to having a healthy relationship. If the person is resisting your attempts to guilt them into apologizing, it's time to turn on the drama. Cry, yell, stomp around, whatever you feel like doing—eventually they'll probably be so desperate to calm you down that they'll say whatever you want to hear. Play on the other person's emotions, as well. Use words like "disappointed," "selfish," and “ashamed” to fuel their internal guilt.

Summary:
Ask questions to get the person to admit what they did. Bring up other things they've done wrong. Play on their feelings for you. Remind them of some good things you've done for them. Deflect any of their attempts to make the situation your fault. Amp up the emotions.