Use openssl to encrypt your plain text proxy password into a file, with e.g. AES256 encryption:
openssl enc -aes-256-cbc -in pw.txt -out pw.bin
alias myproxy='PW=`openssl aes-256-cbc -d -in pw.bin`; PROXY="http://$USER:$PW@proxy:$PORT"; export http_proxy=$PROXY; export https_proxy=$PROXY; export ftp_proxy=$PROXY'
You should source this file into your normal shell environment (on some systems this is done automatically) type 'myproxy' and enter your openssl password you used for encrypting the file done.
The password is available (and readable) inside the users environment for the duration of the shell session. If you want to clean it from the environment after usage you can use another alias:
alias clearproxy='export http_proxy=; export https_proxy=; export ftp_proxy='