appending text to start of file

Hey, was wondering if someone could help me out. I am trying to append a string of text to the start of a file. I know you can easily append information to the end of a file easily when you open the file by setting the mode to "a". The only way I can think of doing this is by making a temp copy of my original file and then add the string I want to append to the start of the original file then append the temp file to the original file. Does anyone know another way of making this work??