quick form question

I have a .php with a simple form in it to populate a table in a mysql db. When the user clicks submit, I check to make sure he/she entered the required info and if all is well, I add the data to teh DB. If he/she left out some info, I echo "You left out some info" and I redisplay the form. When I do this all the info that the user typed originally is gone. Whats the easiest way to put back what he/she already typed? Thanks