There's one really basic trick to getting better, and it's very simple. You need to play people better than you. I played Melee for years with friends, and while we all improved together slowly, I only started to improve quickly when I found better competition. If you try hard and play smart, you'll eventually "even out" to the level of competition you play on a regular basis so really the single best thing you can do for yourself is be a part of whatever strong local scene you can find.
As per how to improve yourself efficiently, the main thing you have to do is look at your game and figure out what exactly is making you lose games. People talk about technical stuff on the boards a lot, and that can be really important with some of the character specific techniques being totally necessary to reaching potential, but there's a lot more too. Do you understand proper stage control, and can you identify which zones you need to control with your particular character in particular match-ups? How tight is your control over your mobility? Do you space your attacks properly (remember to include both X distance and height in your spacing calculations) both to maximize pressure as well as safety? Do you play the ledge game well? Do you maneuver off-stage effectively? Do you properly navigate various stage geography effectively, including slopes, platforms, walls, walk-offs, transformations, pass-through floors, and damaging hazards? Do you properly punish unsafe attacks from your opponents? Do you properly mitigate hits you take from your opponents, including good DI, SDI, grab breaks, techs, etc.? Are you truly using a character who works for you in every matchup, or do you need to either pick up secondaries or switch mains completely? That list is only a beginning of various basic gameplay stuff to be thinking about too, and different players have different abilities at various fundamental aspects of good smash play. Figuring out where you need to focus is huge, and from there, play those strong opponents while pushing yourself to do what you don't usually do well.