Holding in a hit doesnt get you much higher, but does help coat your lungs in loads of tar. Before you take a hit, have a real good long hard cough. It opens up your broncial tubes, and thus the beautiful THC is absorbed quiker, and you dont loose any. But good advice is dont hold it in. If you do, you blow out less smoke than you would if you didnt. Where do you think that smoke is going?