Any object that will be reflected needs to be in your render layer.
After that its just a simple case of going into your reflected object or objects shape nodes and turning off primary visibility on them for that layer.

These are called layer overides, to make a value overidden in a specific render layer just make sure you are in the render layer you want, right click on the attribute and click create layer override.

When you render now the objects which have primary visability overriden will not appear in the render but they will be reflected.
If you want to do a reflection pass which you can then appy to your other layers in compositing just assign a highly reflective matterial or a use background shader to get the correct output (no diffuse or specular color should be visible for this result to be correct).