Before warping a layer via Edit>Transform>Warp, convert the layer into a Smart Object (Layer>Smart Object>Group Into New Smart Object). The advantages are that you can:
1) Re-edit the warping and Photoshop will remember the exact shape of the warp controls instead of starting from scratch;
2) Remove the warp at any time by choosing None from the warp presets to return the layer to its original state; and
3) Edit an unwarped version of the layer by double-clicking on its thumbnail in the Layers palette.
From Photoshop User magazine, 100 Hot Tips issue, tip by Ben Willmore