Long hair mods require you to weight the hair differently.
Usually, I mold my long hair for slim-fitting uniforms so that there is no clipping unless Shep is wearing bulky armors, in which case, the hair just disappears under the armor as though it’s tucked in. It just can’t be helped (unless you’re already way more advanced than me in 3DS Max, in which case, why are you reading my tutorial?!).
If you make long hair mods, be ready for clipping. Here are some additional tips from me, meant to be a supplement to Ottemis’ tutorial: Go check it out! That’s where I learned to make hairs for my Shepard. Ottemis already has a comprehensive and easy to follow tutorial on her website for how to port hairs to Mass Effect.