1. When you are juggled, don't land on stage. But reset to the ledge if you are really high up.
Luigi has poor landing options. His D-air does not hit downward very good, Cyclone is punishable, and N-air is only as surprise element due to Luigi's falling speed. Do a jumped Missile or Cyclone, get above the ledge, fast-fall while airdodging--You should pass by the ledge by there--, SJP back. However, opponents can punish this with D-airs or stage spikes. Another trick is to weave in and out (Not that you can move much) to confuse them and when you see the opportunity, fastfall N-air down!
2. Do footsies correctly.
Assuming I got the meaning of 'footsies' right, always try to SHFF aerials. Retreating F-air and D-air are very good at this. Mix it up with Fireballs. Never dash grab unless it's a punish.
3. Shorthops do well against approaches.
Luigi's shorthop is fairly high. This can be useful to dodge attacks and B-air them right after. Optimal for punishing low attacking dash attacks and dash grabs.
4. Be patient.
There are some attacks on shield that you can never punish no matter what (Ike's F-smash, Ganon's F-smash, Falcon's F-smash, *Insert tons of F-smashes here*). The solution? Let you slide. No. You can't punish. Don't try it. Instead, you can use the slide to your advantage and force some characters to approach you again! (Ike and Ganon, for examples, have a hard time approaching Luigi. They push you back on shield? Be the one who is laughing because they has to approach you because of Fireballs) Some more other characters such as Mega Man and Pac-Man, you'll generally want to stay close to them as you can't force them to approach. Either spotdodge or roll to negate the sliding completely (But these are punishable), or try to fight the slide by dashing or shorthopping, and fire a Fireball if you can.
5. Be passive if you can help it.
No. I don't mean horrible things like letting Mega Man corner you with lemons as you do nothing. But for non-projectile characters (Ike, Ganon, Marth, Falcon, DK, etc), you don't need to approach them. Instead, start throwing Fireballs, both from standing and shorthopping. Make them commit something, and punish OR play it lame, Fireballs again. Against some MUs, it's the best if you just sit by and wait patiently instead of charging in with your hands blazing. Definitely approach against Mega or Pac though... even if Luigi does pretty horrible on that.
I just got less lazier. Mainly because CCI thread is currenty inactive. Ask other Luigi mains for more details, I'm not good at putting my playsytle to words.