In the vein of this question, say I have a 2-qubit unitary gate $U$ which can be represented as a finite sequence of (say) single qubit gates, CNOTs, SWAPs, cXs, cYs and cZs. Now I need to implement a controlled-$U$ gate and hence need its decomposition in terms of simple one- and two-qubit gates. Is there a standard method to get this decomposed circuit?
In Nielsen and Chuang they deal with the general recipe for decomposing $C^n(U)$ gates where $U$ is a single qubit unitary, but not where $U$ is a $k$-qubit unitary cf. glS's answer.