I am having parathyroid surgery tom'w. As someone who is still healing from an anal fissure, my biggest concern is post-operative constipation. Hints?

Preventive measures. Ways to avoid post operative constipation include drinking plenty of water daily, taking a fiber supplement to keep your stools formed and soft, using pain medication sparingly, use stool softeners as needed, and remain active by at least walking daily.
Miralax (polyethylene glycol) Miralax (polyethylene glycol) is a very safe and good laxative in these settings. It is over the counter and should help you.
Be safe. After surgery be sure to drink plenty of liquids. Minimize the use of narcotic pain medication if possible. After parathyroid surgery one may need to watch to be sure their calcium does not get too low, which can aggravate constipation too. Use laxatives if needed and fiber supplements may help. Stool softeners may help a bit.
Post op constipation. This come chiefly from narcotics and calcium. Do your best to keep the narcotics to a minimum and transition to a NSAID as soon as you can and your doctor approves. Take a stool softener in anticipation. You may want to use calcium citrate instead of calcium carbonate, as the former is easier on the GI tract.