This is a question without an easy answer.
Before you go any further, you have to know your character. This is really important. Each character has different moves that are good for different scenarios. Understand what each move does, and when to use it. You need to know when a move should be used and when you should avoid using it at all costs. This is character specific stuff, so check the Kirby subforums for more on this.
Two major things to talk about are risk vs reward and spacing.
Spacing is super important in smash. Extremely important. Some character live and die by their spacing, (Marth) but it's important to have good spacing for everyone. It's safer if you hit with the very tip of your attacks. It's harder to punish, whether on block or whiff. It also limits your opponents options. If they've got a projectile, they can freely spam it if you're far away. If you're close, they can't do that. Projectiles tend to have much longer startup and ending lag then their damage would suggest. Because of that, they can't be used as freely at close range, or you'll get punished badly for it. Find out what your characters optimal range is, and try to keep your opponent at that optimal range. This is partly matchup dependent.
Now for risk/reward. Before you do an action, ask yourself, "Is this worth the risk?". High risk moves should be avoided, generally. You should try to eliminate risks from your gameplan as much as possible. Obviously, you can't just completely remove taking risks, but you can minimize how risky something is. Remember what I said about projectiles earlier. They're much riskier and have lower reward up close, because of the higher startup and ending lag. You don't want to be throwing out projectiles up close. And that goes for other slow moves as well. Much higher risk than you generally want. Don't throw out smashes (or other slow stuff) unless you can be reasonably sure they'll hit. On the other side of things, moves with quick startup and quick ending lag are good because they aren't much of a risk. You can throw those out without being as concerned about whether or not they'll hit. You still need to take into account the risk vs reward of the move, though. Know when to take a risk, and when to avoid it.
Keep spacing and risk/reward in mind, and apply them to your gameplan.
Next is habits. Both yours, and your opponent's
Pay attention to your opponent during a match. Watch what they do closely. If they dash forward, and then roll back a lot, make a note of that, and the next time they do it, predict the roll back and punish. Just some examples. Try to find out what habits your opponent has, and try it capitalize on them as much as you can. Try to also see how your opponent reacts to what you do. If you short hop towards them, see what they do. Try to bait out a response with movements like that, and then punish them for it.
Try to not have habits of your own. Don't rely on the same couple of strategies to win. It gets predictable, and easily counterable after a while. Mix stuff up. Eliminate habits as much as possible from you gameplay.
Be cautious. Don't just rush in blindly. Think about what's happening. Have patience. A common tactic among low or mid level players is to use smash attacks by the ledge to cover ledge options. And it works against people who rush in to things and don't have patience. It gets them destroyed against someone with patience.
Confidence is key. Don't lose hope. If you lose badly, or just lose at all, don't let it affect you. But don't be overconfident. Respect your opponent. Too often people lose confidence in their actions, and just end up doing much worse then they should be because of it.
And above all, play smart and don't get hit. Understand what options both you and your opponent have.
Edit: Let me know if any of this doesn't make sense. This was just sort of a disjointed ramble on stuff. I can clarify if you need me to.
