i had serious lags in the game a while ago and decided to tweak my windows settings a little bit. Since then i have no lags anymore (i mean personal network lag between my client and the server, not stuttering / freezing screens or general lag everyone has at the same time).
You can tweak your TCP settings via the registry or command prompt or you just use this tool.
The tool just changes the network parameters of your OS. It's not one of those "bling-bling-tools" where you click one button and then the "magic-bar" runs to 100%, so you need some little understanding what you can do with it.
There is a FAQ and step-by-step guide that explain the settings. If you are unsure, you should just try out the "gaming tweaks".
This are my settings (i marked the tweaks that seem to have the most impact to solve the lag):
Before you try something, please backup your current setting via the tool.

Some of the settings depend on your personal network-connection (not the gaming tweaks), so don't just copy/paste.