a tip for defending corners; lower your attack/defence as soon as the ball goes over the line, by holding L2 and pressing R2. each time you press R2 the small coloum next to the box that has the players name will go down a colour, with blue being the highest defence level. what this does is instruct all players to drop back and stright up defend and do so by simply crowding the box and the area just ouside it. this can also be used when attacking a corner but this time hold L2 and keep pressing R1, with red being the highest level of attack. this packs out the opposition's area and gives you plenty of targets to get that cross to and allows a CB to get in on the action. - but be warned, with all ur men foward for a corner you are open for the counter big time, the same goes with defening in this manor, with all you men back you may intercept the the ball easily but have no way of going foward so use this accordingly, but with a bit of practice you will learn to judge how hi/lo to set your attack/defence for each situation.
furthermore, i have adopted a high back line (set to A for both), that is positioned right up the arse of my midfield and i use the manual stratergy 'offside trap'. Again this took me a while to get used to but what it does is as soon as it is activated, rushes ur back line forward with great speed. so for instance as soon as i see an attack about to be made against me, i wack on 'offside trap' and my defence is right there with my midfield, totally packing out the final third and not allowing alot of space for the opposition. i have pretty much mastered this technique i like to think against the CPU, of course this is by no means an 'un-beatable' defence and yeh, sometimes the slippery little bastard gets through and has a clear run at goal, where all you can do is hope ur keeper makes the save. But that is what happens in football, you give away goals, and defending is without doubt the hardest part of playing pro.
Anyway, to wrap this up, my main advice is learn to recognize the danger as soon as possible and have a plan of action and stick to it. There is only one way to be un-beatable, that is to never take part.