I'll preface this by saying that I generally suck at breaking throws, but, I have noticed the following:
-Mashing A or B while guarding against a throw happy opponent basically lets you get a free throw break 50% of the time if they are throwing randomly. The majority of online players only ever seem to use the B+G throws, so, mash B while guarding against them. Occassionally, you'll find someone who thinks they are being tricky and does only A+G throws (or are used to say SC2's default layout, where there was no B+G throw button and A+G was so much easier on the thumb to do). I find mashing both A and B alternatingly doesn't work very well, though, that could just be me.
-If you use button binding, then any button bind with an A in it will only break A+G type throws. Any button bind with a B in it will break B+G type throws (if there's no A in the button bind as well). So, A+B is the same as A for the purposes of throw breaking. The Apprentice is the exception to this, who's A+B can break any throw, including low throws. Button binding does make it easier to mash a throw break with those shoulder buttons and not have to use up your thumb holding block.
-Pay attention to ring out throw potential. The Apprentice/Nightmare/Yun-Sueng are almost certainly going to go for a B+G throw to ring you out if their back is to the edge. Raphael is probably going to do a B+G if your back is to the edge. Course, this is a good opportunity to try to trick your opponent and use the other throw, hoping they are trying to break the ring out throw.
-Command throws can be just blocked as normal (I think this is always true). So, don't try to break say Nightmare and Siegfried's low command throws, you can just block low and not have to guess.
-The actual throw break is supposed to happen when that blue circle thing appears (at least, that's what it seems like) and there's a little sound that happens. Though, I can almost never break a throw on reaction to seeing that in a real much (sometimes though, I'm still working on it).
-Some characters have a throw that is vastly superior to their other throw. For example, with Cassandra, I tend to use her B+G throw more, since it leaves my opponent on the ground, very, very vulnerable to whatever mixup I use next on them, while A+G does less damage and has no follow-up potential (though it can ring out, and creating space isn't always a bad thing). But, if I notice you breaking my B+G throws consistently, I'll start trying to 50-50 it up.
-The 2nd half of throws like Sophitia's B+G can be broken. So, if you get hit by the first half, you can still try to break the second half of it via mashing the break.
-I believe setting the training mode dummy to throw, will alternate between A+G and B+G throws. You may find it easier when initially learning, to set up a recorded command of you repeatedly doing one type of throw, so you don't have to guess what's incoming and can get the actual procedure/timing of a throw break down.