There are so many great times in a person's life when you might want to surprise them with a party. It could be to celebrate their birthday or could be as a congratulations for a person who is leaving to follow their dreams at a university on the other side of the world. One of the most difficult parts of planning a great surprise party is the task of keeping it secret from the person that you're putting in all of this effort for. There are some things that you can do to help you out even if you're dealing with someone who usually is able to see through your sneakiness.
If you want to keep the party from being discovered by the guest of honour then the first thing you need to do is make sure that everyone that does know about the event is aware that it is a secret. When people are going to a birthday party at someone's cottage or at a restaurant downtown, it is natural that they are going to want to talk about it before the event. So, right when you open this topic of conversation with people or when you're sending out invitations, you need to be completely clear that this is all meant to be a surprise. If you're sending out online invitations then there are some templates that make this fact abundantly clear and should keep friends and those at their office from accidentally spilling the beans.
Plan something with an accomplice for what the guest of honour will think that you are doing during this time. It is very rare that a wife would forget her husband's birthday and the person you're trying to surprise might suspect something if they just announced that they are pregnant and there is no word about having a shower. To throw them off the scent you should find someone else in their life that can help you arrange a fake dinner out or afternoon at the spa that sounds like something you would do on a regular day. Making this fake plan as real as possible will help keep the surprise under wraps.
Be clear about times that you want people to arrive and what you want everyone to do. You might have people bringing in flower art in the afternoon while your wife is at work or driving the new car into the garage while your sixteen year old is still at work. If you are very clear at the beginning about what you want and when the party starts then there will need to be less communication and questions answered as the party approaches. That means less chances for the guest of honour to find out about the event before they arrive at their home that evening.