The most important way to keep the bitterness out of sautéed spinach is not to overheat the olive oil or burn the garlic. Both of these things will cause your spinach to be more bitter. Also, the freshest spinach won't be bitter so try and buy local when you can. Store-bought greens almost always have a hint of bitterness you come to know with any kind of greens, but locally grown is a whole different story.