Yo-yo comes out fast and is great for shield pressure if you hold it on a shield. (this can even break them if your opponent is a scaredy cat)
PK Thunder is very useful for gimping recovery, spacing yourself, and works great on sword characters (soome of Ness' worst match ups)
Delaying your fall with PSI Magnet can mess up a combo, and can even make your opponents string your string. It also has a very nice windbox, so use that how you will.
Downthrow to shorthop f-air to f-air to PK Fire works well at lower percents/heavy characters
Up-air has a large multi-hit hitbox and is great for juggling characters with poor aerials. (Use it while falling as well!)
His down-air spikes, but can be a little hard to time
If you can control your PK Thunder well, you can intentionally (or accidentally lmao) miss your recovering opponent and hit yourself with the thunder for a great kill move. (just be careful not to blast yourself off into freefall)
PK Fire is unreliable on characters like Pichu (light), Jigglypuff (light and floaty), Little Mac (can f-smash through), and Megaman(Leaf shield).
Be very careful with your spacing. Sword characters can easially thrash a Ness if they aren't careful. Learn your opponent's attack patterns. Be as adaptable as possible.
Short-hop is your best friend.
Save. Your. Second. Jump.
Neutral-air is one of the (if not the) best OoS options for Ness.
Since you're so floaty, try and go for ledges when you recover.
Don't get tilted and spam PK Fire. If you're getting sick of... say, projectile spam; and you counter with your own spam, PK Fire will probably not come out on top. Keep your cool.
And finally, dont forget to have fun playing as one of the most 'accused-of-spamming' characters in the game!
EDIT: Forgot to mention to keep your backthrow as fresh as possible, and to not underestimate a forward throw.