Linux Persisting Proxy

With following changes in system file I am able to solve my issue with system Ubuntu 11.10 for updates.

For proxy setting here is steps:
1) go to file:    vim /etc/apt/apt.conf
2) Put this lines in this fileAcquire::http::proxy "http://myusername:mypassword@<ProxyServerName>:<Port>/";
Acquire::ftp::proxy "ftp://myusername:mypassword@<ProxyServerName>:<Port>/";
Acquire::https::proxy "https://myusername:mypassword@<ProxyServerName>:<Port>/";
Save it and close.

3)Then update system.

I hope it will help other also, so i am sharing information with you.