Okay there are lots of things that need to be tuned up and stuff, but I'll name the essentials:
You've absolutely got to L-cancel! This is crucial in Falcon's game, especially after a Dair. It'll definitely help out your comboing ability. SHFFL everything! We should see you comboing Pikachu and Link across the screen!
You only wave dash and dash dance when your opponent is dead? You've gotta learn how to incorporate those assets into your mindgame. Falcon has the best DD in the game, use it to your advantage.
Don't roll too much! Falcon has an incredible dash that can help you out with spacing and evasion more efficiently than rolling.
Don't use your double jump unnecessarily! If someone hits you during your DJ, then it could mean one stock. We all know how horrific Falcon's recovery is.
These are some of the major things I noticed. If you're having a hard time comboing, then try to keep it simple. Master the G-reg combos (throw to knee).
Also, when approaching your opponent, you don't necessarily have to attack. Why not trick him into doing something he doesn't want to do? For example, you short hop toward your opponent, he shields, and as you get close, he tries to grab you, but you've wavelanded away from him. From here you could probably do a throw to knee combo for a stock.
Just simple things like that could really improve your game.
Sorry if this is waaay too long, lol. =]
Hope it helped!
