the best advice there is to be less predictable, and to be more aware of when your aerials are unsafe. i think at least one of two things is happening.
1. falcon is kneeing in place and it's going through your attack. they know their knee will beat or at least trade with anything you do. all they have to do is figure out (or guess lol) when you're going to be in range of it. so your best bet is to let them miss and punish THEM for being predictable.
it's kind of like when a jiggs player predicts fox is going to run at them and they fsmash in his direction. if they were right, it probably worked out. if not, the jiggs main then got punished. the difference of course is that falcon's knee is an aerial, but any lag is usable in melee.
2. he runs around then suddenly comes flying after you and knees you from halfway across the screen. in that case, the problem was probably still predictability. he kneed where he thought you were about to move. you could have moved there and shielded, or ducked, or just not moved there at all...
a little more specifically, my guess is that you're jumping too much. jumping is kind of bad against falcon. he wants you to be in the air 'cause you're so much easier to hit that way. jumping makes you vulnerable to things like random knees

so you should move around with wavedashing more than normal.
falcon has so much more trouble dealing with grounded jigglypuff. go play a match with falcon vs. a jigglypuff, you'll be constantly waiting for them to jump while being slightly away from you so you can knee them. it just feels so right. lol
crouch should also largely replace your shield against falcon. shield blocks aerials but can be grabbed. crouch blocks both.
edit: look at my post count :O