1. Every time you login check the last login activity for your account. You can see the IP address of the computer using which your account was logged in last time.
2. Donot login to your gmail account from public places. Even if you do make sure you do the following also :
- Donot click on the remember password option
- Make sure you log out before you get off the system for any reason.
3. Enable two step verification. If you use this system, every time a person login to your gmail account you will be first asked for your normal password that you use for logging in usually and also if you enter it correctly a verification code will be sent to your mobile phone via SMS and only if you enter that code you will be allowed to login. I would recommend everyone to use this system as a highest security system in gmail.
For enabling that system go to your google accounts settings page nad there you can see an option called 2 step verification. Click that and then enter all the details that they ask for and save it.
4. Every time you login check for the URL. Donot use the links that someone gives you to login. Everytime login from gmail's official page and not from any other pages.
5. And also as usal most of the people donot know real hacking and they just hack your account by just guessing the password. So choose a strong alphanumeric password.
And thats it. Hope this helped you in making your gmail account more secure. :)