Also, remember that each Smash Bros. game play completely differently due to game physics. Smash 64... well, you said you were great at that, so I won't say anything (not to mention I never play it myself, so I wouldn't be able to offer anything on that subject). Melee is more fast-paced, combo oriented, and requires the knowledge of wavedashing to get good at it, which takes time and practice to master. Brawl is more centered around punishing and predicting. While the former two also has that prediction-based play, Brawl takes that element and pretty much centralizes it, especially with a smaller hit-stun window, you have to predict what your opponent will do after you hit them and try to punish their responses.
With that said, everything else I can say has already been said; just practice. You can also go on Youtube and look at various combo videos for guidelines as well, though don't overly rely on them as "that is how a character should be played", since some combo vids tend to go for flashy out-of-the-way kills that are fun to watch (at least in the case of Brawl, anyway).