It's possible to export existing sketches but a problem with this route is that if you have any construction lines they will be exported as well.  In most cases you won't want your construction lines exported. The workflow you'll want to follow is to create your sketches and extrude all your parts.  In my case I simply modeled my cell phone holder exactly the way I want it to look in its assembled state.

Click on the Sketch menu and select Create Sketch and then click on the surface of the body you want to export.  This will create a sketch on that surface and you should see a new sketch icon on your browser. Now simply click on Stop Sketch on your toolbar.  It doesn't look like you did anything but the sketch you just created captured all the geometry from that body. You can verify this by hiding all the bodies and every sketch besides the one you just created.  Hover your mouse over the sketch and you'll see all the captured geometry.

Right click on the new sketch you created and select Save as DXF.  You can now import the dxf file into whatever software you use to send the part to be laser cut or CNC'd.  In my case, I opened the file in Adobe Illustrator and printed to an Epilog Laser Cutter. Other popular software used with laser cutters are Corel Draw and Inkscape.

Here's a video of the entire process.  Learn how to design your own creations for 3D Printing other desktop fabrication tools by visiting desktopmakes.com.