To configure proxy settings for apt-get or aptitude or synaptic manager, all you have to do is edit the file, /etc/apt/apt.conf .
Here you have to mention the following 3 lines::
vignesh@vignesh-laptop:~$ cat /etc/apt/apt.conf
Acquire::http::proxy “http://username:password@host:port”;
Acquire::ftp::proxy “ftp://username:password@host:port”;
Acquire::https::proxy “https://username:password@host:port”;
Once you have made the above changes in /etc/apt/apt.conf , you will be able to use apt-get, aptitude or your synaptic manager through the proxy.