sensitivity "3"
This changes your mouse sensitivity... pure preference obviously. The only tips I can suggest is don't put it over 13 on 640x480, 10 on 800x600, 8 on 1024x768, or 7 on 1280x1024, or 5 on 1600x1200 or you'll for sure lose single pixel accuracy. You want to avoid pixel skipping if at all possible.
m_filter "0"
Although the concept of smoothing mouse inputs over the last two frames sounds attractive, this is something you want to have set to 0. Basically by averaging your last two inputs you are watering down everything you do with the mouse. You know what to do better than the computer does. This adds a slight bit of mouse lag too. I would only recommend using this if you have a mouse that is crappy and jitters alot.
m_pitch ".022"
This determines how sensitive your mouse is on the vertical axis. When you look straight ahead the most you need to turn vertically is 90 degrees up or down, but the most you have to turn horizontally is 180 degrees left or right. This means you could potentially lower this setting and gain precision with aiming height and recoil control. Set it however you like. I use ".0165" which is 75% the default speed. Adjust in small amounts like +/- .005 and set to your preference.
m_yaw ".022"
This determines how sensitive the mouse is on the horizontal axis. If you want to adjust your sensitivity in only the horizontal direction change this in small amounts like +/- .005. Set to your preference.
m_customaccel "0"
This allows you to define a custom acceleration curve in CS:S, you don't want this so turn it off.
zoom_sensitivity_ratio "1"
This changes the sensitivity in relation to normal when you use a scope. Raising this increases the sensitivity when scoped, decreasing this setting lowers the scoped sensitivity. The setting of 2 is as fast while scoped as not scoped (like quake 3), anything higher than 2 then is a pointless setting. Many people set this between 1 and 1.2. Set it to your preference.