Some good advice

If you fancy experiencing the excitement of the whole thing, remember that:

It's no place for kids - they could get stepped on and at their height there is not much air moving round.

It's no place for wearing sandals or light shoes - you're going to get stepped on and also many people drop their empty champagne bottles on the ground when they have finished them, so broken glass is to be found all over the square.

Don't wear any clothes that you value - and this is good advice for the whole of the Fiesta.
Don't take your camera or any valuables with you. Taking decent photos will be impossible with all the liquid spraying round. And if you drop anything like your wallet or whatever it could be difficult to look for it among the packed crowd. 

If you don't want to see it inside the plaza, you can just do the same as many of the locals do, and watch it from the T.V. in some bar or other or at the big screens at the Plaza del Castillo.

By the way, there is usually some kind of "alternative" gathering to celebrate the "txupinazo" held in some other part of town (at the same sacred hour of course) You know how it is - some group or other who's cheesed off with the Mayor and his party for some understandable reason or other. Maybe not a bad idea to join them.