The absolute best way to work is create the document using only vector shapes, layer styles and (if you have to) Smart Objects that are high enough resolution to cover the @2x size.
If constructed properly, your document can be scaled up and down with no loss at all — Photoshop will rerender shapes and layer styles at size, so they'll be as good as possible, with two caveats:
Layer style properties have values that are integers. Scaling down will round them. A blur radius of 3 will become 2 when scaled down 50%, because 1.5 is not possible. Oh, and make sure you have Scale Styles turned on when you scale the document.
Shape mask feathering isn't scaled when you scale the document.
If you need textures, these can be done with pattern layers (with a vector shape mask), or a pattern layer style, or a Smart Object that has a vector mask.
You should avoid bitmap scaling at all costs. This means you shouldn't ever scale the final PNG assets... you need the Photoshop document to be built properly and you'll need to re-export all assets.
Here's a comparison between two common forms of bitmap scaling and scaling properly using vector shape layers and layer styles.
I've written more about designing for Retina here:
Designing for Retina display