Remember to backup your original copies in case you want to revert.
1a. Download: MasterServers.vdf (for HL2, Steam, CS:S, TF2, L4D, L4D2, Orange Box, ...)
1b. Download: MasterServers.vdf (for CS1.6)
1c. Download: MasterServers.vdf (for CS:S 1.0.0.34)
2. Copy MasterServers.vdf to config directory
* Standalone: C:gamesCounter-Strike SourceplatformconfigMasterServers.vdf
* Steam: C:Program FilesSteamconfigMasterServers.vdf
* NOTE: If masterserver2.vdf is in the same directory then overwrite it too - notice the naming difference
3. Make MasterServers.vdf read-only
* Right-click over MasterServers.vdf, select "Properties" and check "Read-only"
Making the VDF read-only prevents your game or Steam from overwriting it in some cases.