How can I prevent the blood clot from coming out too soon after a wisdom tooth extraction?

Common Sense, Really. The first thing is to leave it alone. Don't mess with it, don't play with it, don't brush it, don't swish vigorously around it. Also, avoid tobacco, alcohol, & nicotine, as these can cause the clot to dissolve and be lost. Avoid hard, crunchy foods. If you lose the clot, healing will be impaired & delayed, and it might be quite painful. So, just give it time and allow your body to heal properly.
Postop Instructions. Avoid vigorous rinsing, smoking, and spitting (females do not spit, they expectorate). Avoiding carbonated beverages may be advised by some offices. More importantly, avoid calling your dentist or surgeon bad names as this increases the risk for the loss of the blood clot, or a dry socket.