We present a simplification of Lieb's proof of the flux phase conjecture for interacting fermion systems--such as the Hubbard model--at half-filling on a general class of graphs. The main ingredient is a procedure which transforms a class of fermionic Hamiltonians into reflection-positive form. The method can also be applied to other problems, which we briefly illustrate with two examples concerning the t-V model and an extended Falicov-Kimball model.