  Perturbative calculations in field theory at finite temperature involve sums
over the Matsubara frequencies. Besides the usual difficulties that appear in
perturbative computations, these sums give rise to some new obstacles that are
carefully analized here. I present a fast and realible recipe to work out sums
over the Matsubara frequencies. As this algorithm leads to deal with very
cumbersome algebraic expressions, it has been written for computers by using
the symbolic manipulation program Mathematica. It is also shown this algorithm
to be self-consistent when it is applied to more than one loop computations.
