Space with Fairs, be careful of shielding and crouch canceling though. Sometimes if they shield a DJCFFL Fair you can grab them out of their shield if you're close and/or quick enough.
Wavedash -> grab is (IMO) the best way to get grabs since Ness has such tiny arms. All of Ness' throws are good except for downthrow which can be DI'd away pretty easily. Maybe at very low percents it's good for a tech chase, but I usually use Upthrow onto platforms if I want a tech chase. Uair is great for chasing them around platforms.
Work on getting REALLY tricky with your DJC momentum changes. Out of the 4 DJC capable characters, Ness arguably makes the best use of DJC. Try practicing forward SH -> backwards DJC aerial -> wavedash away after landing. With Ness, you NEED the mind games so always try to mix up your movement patterns.
Also, remember that PK Thunder from below shield pokes tall characters standing on platforms like Marth, Falcon, and I think Sheik and Peach. If you have center stage, tech chasing PKT is still hard, but you can rack up a bit of damage and humiliate your opponent while you're at it.
At higher percents, Dair to Bair works just as well as Falcon's Dair to Knee. I love that combo. At lower percents, try to get Dair auto-canceled if you can and follow up with a string of Uairs DK style, or even try going for the re-Dair.
Nair is good for GTFO situations.
PK Fire edge guarding is literally pwnage. Against spacies, I love catching side-B with a PKF and watching them fall or playing a bit of T-ball while they're burning if you know what I mean.
PK Flash: The ultimate disrespect. But if used with perfect timing against lame recoveries like Falcon or Ganon, it's actually really good option.
Upsmash Yoyo, YYG aside.... I guess this is an OK move. Catches them off guard sometimes.
Downsmash Yoyo, like Simna said once "It's safer to taunt". This is true.... there are less inactive frames on Ness' taunt than on Dsmash.
You pretty much know the rest.
[EDIT: Vrud likes this, that means it's good advice.
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