Using the QC command $maxverts 64000 at the top of the file, you can get a $bodygroup or $model to compile up to 63,999 vertices without the compiler splitting it. Note that any value higher than 64000 will be ignored. I often set it to 999999999 just because it's easy, but it hard-caps at 64000. The value is exclusive, so 63,999 vertices will be one model; 64,000 vertices will get split.

Beyond that, you need to split meshes into separate $bodygroup and $model blocks. Ideally you want to avoid splitting anything that has morphs or flexes on it, as that just complicates things immensely. There are ways to get flexes to work correctly across split meshes, but the less you have to do it, the easier it'll be for you. So for example, for high-poly character faces, I will often split the top of the mouth internals (gums and teeth) and the eyeballs into a separate $model, so that they're not included in the main flexed mesh. Because the top of the mouth should never move (anatomically it's hard-affixed to the skull), and Source uses eye shaders so the eyes don't need to morph either.