Crashing parties is pretty rough because you need to be accepted for it to really pull off smoothly, and it really isn't that hard. Just go out and meet new people, like supremo and zubs and others have said.
But hey man, this is important.....you are probably approaching parties (even ones you are allowed to go to) with a totally wrong mindset. It's never good to go in saying "Oh this is the worst thing that could happen, let me prepare myself..."
Doing something like that is an easy "out." You don't want to get disappointed, so you already assume the worst. Take it from me, if you do that, you'll just end up avoiding everyone at the party and be all by yourself, because you internalized this worst case scenario and you are trying to avoid it.
So the first thing you do, before any other thought process, is get rid of a negative mentality. In science, and in life, likes attract. Negative thoughts attract negative situations.
Apply that to your life OUTSIDE the party too. Like as zubs said, try the lunchroom. You probably assume the worst and internally feel "correct" when the worst does happen. Its really hard for a group of people, in the lunchroom, to say something mean to a guy with a smile on his face.