what should I use to clean my plate after printing with PETG?

I'm not using anything specific for PETG clean up. IPA to get rid of oily residue and soapy water from time to time.

I use textured PEI, and this is pretty much my regimen for PETG as well. So far, a thorough 90% IPA wipe with a lint-free towel or microfiber right before each print is all I have ever needed to get good adhesion.
Honestly this is the way for pretty much all surfaces, exception if the surface tells you not to use certain cleaners (I have one that says to not use soap, only IPA).

I've never really thought much about something specific for petg.

I use a magnetic pei bed, both textured and non textured with a glue stick layer.

I don't often need to clean but I'll use soap and water in the sink for the majority and then a isopropyl squirt and rub with a rag. Then I reapply glue stick.

This lasts a lot of prints for me for pla, petg and abs.

Really, I'd think you would need to avoid chemicals that would react with your bed and make sure there is no residue left by your cleaner. For my pei bed I bet I could even use acetone. Beyond that... Happy washing.

There are lots of internet gudes on this. Here is one random one I just found. https://www.wevolver.com/article/how-to-clean-3d-printer-beds-glass-pei-adhesive

Why do you ask? Are you seeing some sort of residue? I've never heard of PETG leaving anything behind, just being REALLY stuck and pulling up some of the surface, I think. I've no personal experience.

No residue just some really stuck on bits, and I'm using a PEI sticker so I can't really get it off super easily. Will probably invest in one of those magnetic beds soon.

I found that I had to use gluestick for PETG on my bedslinger - without it the print wouldn't even stay in place for more than maybe a 60 minute print. I have heard about PETG fusing to glass beds also, though I have had the opposite problem, and I would just clean it off with some alcohol, but there didn't seem to be much of any kind of residue.

Now, the gluestick, on the other hand, that is a nightmare, until I found the method: spray it with windex or water, let it soak in for thirty seconds or so, then scrape with any spatula-like object. I often take two passes, but wetting the glue turns it into a looser sorta paste that comes right off. Once all the glue is off, go over it with alcohol to dry it all up, clean as a whistle.

My goto on a glass bed was blue painters tape applied with a spatula or flat tool. I discovered that the natural oils off my fingers transfered to the painters tape when applying making the first layer fail to stick every time.
- PETG is a diva. If something's not perfect, it refuses to print. My CR-10 had the infamous first version of the Creality bed springs, and vibrations would cause enough movement of the tramming springs, the print would fail. Even watching it put down a perfect line of filament, it would move and fail during the first layer on a larger print. My problems mostly went away after upgrading bed springs.
  
  Also, I live in a swampy area, I have to print PETG out of a heated dryer set to 55 degrees all the time. That took care of almost all of the other problems.
  
  PETG is softer than PLA, so the extruder "bites" it differently. Calibrating my e-steps for PETG was the final key to printing great PETG parts.
  
  I still think ASA is easier for me to print than PETG.

I have printed lots and lots of parts in PETG and not once did anything special to the bed after a print.

PETG and PEI are fundamentally "too compatible". If you have a smooth sheet of PEI (like the ones on the Prusa), PETG can fuse permanently to it. Lay down some a thin layer of glue stick as a release agent before using any PEI surface.

Regarding other tips about PETG: make sure the filament is dry. Wet PETG will pull lots of strings, no matter how "good" you're tuning your retractions.

Is this just down to the humidity?
- It happens faster with high humidity. But it will happen in drier air too. A dry box will keep PETG dry, but not necessarily dry it.
  
  It's not uncommon to have PETG with problematic moisture content directly from the factory, even shrink wrapped with desiccant.
  
  I print PETG from a dry box set to 55C. Even when my print is done and it sits for a weekend (rare), I keep the dryer on. If I have to swap a spool, it goes in a dry box that I keep below 20% RH. I also use a lot of cheaper PETG, though. It's my least favorite filament to print so far, but one of my favorite to make functional held objects since it's one of the densest polymers used in printing and feels heftier than most 3d printed objects.