1. Use multiple user accounts. Every modern OS has it, so bloody use it. Letting other people use your user account is like letting other people use your safety deposit box: it's just stupid.
2. On that note: don't give administrative permissions to those who don't need it. That's like... well, the most obvious example isn't completely forum-friendly, so I'll leave it to your imagination
3. Don't stay logged into websites: use a password manager.