Statement

I am not a security expert but I have implemented a few of these measures in my Ubuntu box and so thought about discussing about it. Please know that implementing these features will not make your box fully secure. You can discuss or find solutions for any security issues in the Ubuntu Forum

The two features I will discuss is ssh, as you let peoples walk in your machine through these applications.

SSH( Secure Shell Access) normally use password authentication to let peoples in. Firstly you will make up your mind if you at all need to have a ssh connection to your machine. If not, you can disable ssh from the Synaptic Package Manager. If you need the ssh connection to your machine, then change the port for it. Default port for ssh is 22 . Use the following method to change the port

1. From your terminal session, edit /etc/ssh/sshd_config
# vi /etc/ssh/sshd_config

2.Look for the following line:
#Port 22

3. Change the line so it looks like this:
Port 3178

4. Save and close the file

5. Reload the service.
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Reduce the Amount of Grace Time

In the same config file look for the line
LoginGraceTime 120

Change it to
LoginGraceTime 30

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Disable Password Authentication

To disable password authentication, look for the following line in your sshd_config file:
#PasswordAuthentication yes

replace it with a line that looks like this:
PasswordAuthentication no
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Key Based SSH Login

To know all about generating keys, please read the wiki

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You have now a secure shell access to your machine.