Lower Texture Resolution:
Here are the settings for optimized textures regarding the Face, Body(tattoos), Clothing(designs). These can be around the 1024x1024 resolution and some can even be down to 512x512 or lower. Another thing you can do is reduce the amount of textures you're using.
EX: Black shoes do not need a separate texture/material and can use a Black material used for other parts of the avatar like metal pieces or other clothing.
Limit Total Materials:
Like said above you should reduce the amount of textures and materials used, by using the same material for similar pieces of the avatar. As a side tip for those who do not like the way Blush looks with the limited texture options, you can remove the "Blush" blendshape in the gesture animation(s).
Limit Bone Count:
The limit is 8 Physbones on an avatar, this is enough for some Hair, Tail, and Ears. If you would like more bones consider PCVR.
Further questions or more in-depth answers can be found on the Official VRChat Doc page. Here