Yes, you can reuse variables. This is how you do it:
SELECT
@total_sale := s.f1 + s.f2 as total_sale,
s.f1 / @total_sale as f1_percent
FROM sales s
Read more about it here: http://dev.mysql.com/doc/refman/5.0/en/user-variables.html
[Note: This behavior is undefined. According to the MySQL docs:]
As a general rule, you should never assign a value to a user variable and read the value within the same statement. You might get the results you expect, but this is not guaranteed.