  We present a prescription for an effective lightcone (LC) Hamiltonian that
includes the effects of zero modes, focusing on the case of Conformal Field
Theories (CFTs) deformed by relevant operators. We show how the prescription
resolves a number of issues with LC quantization, including i) the apparent
non-renormalization of the vacuum, ii) discrepancies in critical values of bare
parameters in equal-time vs LC quantization, and iii) an inconsistency at large
N in CFTs with simple AdS duals. We describe how LC quantization can
drastically simplify Hamiltonian truncation methods applied to some large N
CFTs, and discuss how the prescription identifies theories where these
simplifications occur. We demonstrate and check our prescription in a number of
examples.
