this is an example:
(windows only, 1.5.2 only)
@echo off
title Minecraft Offline
echo Minecraft Offline Launcher
set mcinstall=C:\Users\****\AppData\Roaming\.minecraft\bin
set usrname=*enter somthing like TrueMU*
cls
cd %mcinstall%
echo Launching Minecraft with username %usrname%.
start javaw -cp minecraft.jar;lwjgl.jar;lwjgl_util.jar -Djava.library.path=natives net.minecraft.client.Minecraft %usrname%
pause
exit
this code should log you in as truemu
