By far the thing that will make the most difference is plugging in a network cable. Even with the fastest internet connection in the world, WiFi can cause issues with live video. You don't notice it with streaming video because there is what is called a "buffer". Basically, it streams the video on a few second delay so if the Wifi messes something up, it has time to recover and you don't notice. With truly live video, we don't have that luxury. The video below (from TalkDesk) has a good explanation.
Here are all the tips we have compiled.
Plug in a network cable
If you can. WiFi interference is the biggest cause of quality issues.
Sit close to the router
If you have to use WiFi, don't sit too far away from it.
Stop other devices using WiFi
Again, if you have to use WiFi. Turn off Netflix, Spotify, etc.
Ensure your browser is up-to-date
Check for updates if you can. We recommend Chrome.
Close other programs/tabs
Close other programs running on your computer if you can.
Plug-in a headset
Bluetooth devices cause a delay. Built-in mics pick up other noises.
Restart your computer/phone
If you have time. It frees up resources for your call.
Use a computer over a phone
The camera is fixed and, generally, they have more grunt than phones.
Use a well-lit room
Video quality is better when there is more light.
Light source in behind camera
Less shadows on your face means you look better.
Have a still background
Don't have lots of movement in the background (like a fan).
Solid Internet Connection
Try and use a connection that is not prone to dropping out.
Sit as still as you can
It looks more professional and big movements use more data.
Keep your background uncluttered as possible.