I practice combo's religiously so that I never lose my timing in a real match.
I go through each character and look at each scenario (aka move) and make a list of every one that can be punished on block, or needs an evasion of some sort. (This took me 6 months to complete in SC4, with 1hr of practice per day.)
I look at every offensive option my character has and match it to every offensive situation they can create (took about 2 weeks).
I then look at every offensive option I have and look at what defensive options each opponent has. (I never finished this for 4.)
I quickly came to the conclusion that Jump -> Doom Combo was broken beyond belief, but yes, like Hates said, if something pisses you off, Lab it and break it apart. Next time they try it on you destroy their soul. Finding out you have a universal answer to every mix-up option of a move is key.
And that answer doesn't have to be a single move, it can be an option select such as Step-G.
SC5 is possibly going to be far more Training modish since you'll also want to practice timing your Just Guards for certain moves. GI has always made some options avoidable, but Just Guard makes them purely unsafe. You can Counter GI someone, If they Just Guard you, you're fucked.