I find that a large pair of sharp scissors do the best job because they give a nice clean cut and don't tear the leaves. Hope that's helpful! :o)

I use scissors or secateurs for the wide flat ends if needed.
The best thing I've found for cutting the thick bases of leaves close to the ground is a grapefruit knife. It is great for getting in the tight spaces and has a slightly angled end, with both sides being serrated cutting edges.
Any other leaves that are old and brown and withered may just pull away with some tugging.

I find a carving knife is best though not recommended that you use the same one that you want to carve your Sunday roast with.