I have actually recently been addressing this very issue within my own gameplay. Perhaps I may impart a few words of wisdom.
First off, the key to "intelligent aggression" in my eyes is the ability to create, build, and sustain one's momentum. While a rather abstract concept, momentum can best be envisioned as possessing control of a match and dictating the movement and flow of that match (as I stated, not the most concrete of definitions, really). A significant portion of this relies upon your ability both to read and to react to your opponent. Successfully anticipating your foe will allow you both to stay on top of him or her and to punish any openings. However, this approach requires a good deal of thought, instinct, and proper decisions on your part, all components of higher level play (to which "intelligent aggression" is a gateway, in my opinion).
What you are looking for, Rapid_Assassin, is the ability to read, predict, and punish your opponents, which ties back in to the concept of momentum discussed above. The most I can tell you here is constantly to look for some of the most common openings in an opponent's game. For example, you should be paying enough attention to your opponent to see if, say, his Marth tends to space his fairs incorrectly; here, shield grabs are your friend (or perhaps an out-of-shield option as well, depending upon your character). As well, while on the offense, keep a sharp eye out for some of the most common efforts by your enemy to gain some breathing room. Perhaps he always runs away after he misses an aerial, in which case you should know to pursue. Perhaps he spot-dodges after missing a shuffled aerial, or always full-jumps by the ledge or tries to spot-dodge one of your falling aerials. You should notice that all of these scenarios imply that you are on the offense, pursuing and attacking your opponent, while they are trying to find an answer to your aggression. One of the keys to "intelligent aggression," then, is the ability to find answers on the fly to your opponent's efforts to escape you; this will allow you to stay on top of them, to maintain your momentum, and to play intelligently while staying on the offense.
Technical aptitude is also very much required for a successful offensive game plan. If one wishes to maintain his or her momentum (and thus his or her smart, offensive play), one cannot afford to lose all momentum instantly by getting shield-grabbed, out-spaced while approaching, grabbed from a falling aerial without an L-cancel, etc. You will notice that in all of these scenarios, were the player to have complete technical control over his or her character, he or she would emerge unscathed while the enemy would be jabbed, shined, hit with a spaced aerial, and so on; this would allow the player to continue to pressure his or her opponent, thus maintaining control of the match (momentum) and protecting the integrity of the offense.
In short, then, the keys to a successful (read: smart) offense are both mental and technical aptitude. However, there is one more ingredient to the mix, and that is your own absence of fear. You can never allow yourself to become afraid of your enemy, or that you might get punished if you go in, or what have you. Sometimes you just have to go in there and take what you can get. If you connect, keep your momentum and stay on top of your foe, always on the look-out for those easy openings and attempts to gain some breathing room, and following (key word) as appropriate. If you miss and are punished, DI the upcoming hit or hits, make the proper teching and recovery decisions, and keep on going. No fear.