What some people call 'bad behaviour' in a toddler...

My 2.5yr old sometimes hits, pushes and throws - what ought I to do about it?My 2.5yr hold is normally a very pleasant toddler, very peaceful and calm and non-confrontational. But occassionally she throws things, and sometimes they're heavy things that could do some damage, for no particular reason as well as when she's angry. If we ask her not to do something (and we don't often do so!), she sometimes hits us. When she's going through a bad patch this happens more and more often and quite often includes pushing her baby sister over, or hitting her. She's not a hard work toddler, and I expect the occassional meltdown when life doesn't make sense, and respect her needs as much as I possibly can (she's still breastfed, sleeps in our bed, isn't forced to do anything she doesn't want to do) but I'm finding this unpleasant behaviour very hard to deal with - in fact I haven't a clue how to deal with it! My instant reaction is to snap, or say 'no' very firmly (although ideally we try not to say 'no' to her!) - it makes me very angry to see her hit/throw/push - but I'm very aware that it's probably not the best course of action. I'd very, very much appreciate some guidance or suggestions or anything!

Many thanks