.DS_Store

This will affect the your interactions with SMB/CIFS, AFP, NFS, and WebDAV servers:

1. Open the Terminal.
2. Enter: defaults write com.apple.desktopservices DSDontWriteNetworkStores true
3. Press Return.
4. Restart the computer.

Tweak Safari

Don't have Safari display PDF documents, rather have them downloaded instead:

1. Open the Terminal.
2. Enter: defaults write com.apple.Safari WebKitOmitPDFSupport -bool YES
3. Press Return.
4. Open the QuickTime preference pane in System Preferences
5. Click the Advanced tab, and then MIME Settings...
6. Under the Images list, make sure PDF Image is not checked.

Logitech V270 Bluetooth Mouse Scaling

1. Open the Terminal.
2. Enter: defaults write -g com.apple.mouse.scaling 0.03
3. Press Return.

Optimize Apple Mail

1. Quit Mail and open Terminal.
2. Enter: cd ~/Library/Mail
3. Press Return.
4. Enter: sqlite3 Envelope\ Index
5. Press Return, An sqlite> prompt will appear.
6. Enter: vacuum subjects;
7. Press Return.
8. Press control and D after the prompt appears
9. Restart Mail