Maintain Weight
Maintaining a healthy weight is important to healthy periods. As someone who's struggling to put ON weight, my periods suck. They're highly irregular and painful.
See, if you're below/above average weight, your body has a hard time maintaining a proper schedule/biorhythm for this.
Besides the fact, your period can sap all your iron and cause anemia if you don't get enough. Eat loads of spinach and other iron-rich foods if you're underweight!

As for being overweight... Same issues. Irregularity and pain, so try and lose the extra in a SAFE way (IE, exercise, healthier foods, etc.)!

And here's an added PSA... Don't try and lose too weight to look skinny. Losing healthy weight (overweight -> average) is awesome, but there are loads of issues that come with being underweight-- trust me, I AM underweight.