When you publish your images (File > Publish To > Images), make sure you set the Image Quality to either High or Highest. Also, set the Image Size to Full Size.
Also, check what your page size is by going to Page > Settings and then clicking on the Size tab. In the presets dropdown, there are a number of different choices, each optimized for a particular resolution. A good rule of thumb is to create your layouts at the largest size you're going to think you want to print at (hint - the default of 8 x 8 at 200 pixels/inch will give great results even when printing 12x12 pages).
If you've created smaller layouts, don't worry! They can still print out OK, especially if you are getting smaller prints like 4x6. They just might not look as good as they could if you want to print them at 8x10 or something.
Tip - For truly stunning printed results, you'll want 150 pixels per inch or greater (200 ppi is best - 300 ppi is too much for any consumer level print, but some people choose it anyways).
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