  In this paper we find estimates for the optimal constant in the critical
Sobolev trace inequality $\lambda_1(\Omega)\|u\|_{L^1(\partial\Omega)} \le
\|u\|_{W^{1,1}(\Omega)}$ that are independent of $\Omega$. This estimates
generalize those of \cite{BS} concerning the $p$-Laplacian to the case $p=1$.
  We apply our results to prove existence of an extremal for this embedding. We
then study an optimal design problem related to $\lambda_1$, and eventually
compute the shape derivative of the functional $\Omega\to\lambda_1(\Omega)$. As
a consequence, we obtain that a ball of $\R^n$ of radius $n$ is critical for
volume-preserving deformations.
