“How do I keep food from getting under my partial denture?”

It may be due to a change in gum contours, which is normal over time, but creates a space from the original fit of the partial. Have it evaluated for a possible reline or just try some Fix-o-dent to see if that helps. You may want to consider having implants placed so you can eliminate the need for a removable appliance altogether.

It may not be fitting properly, but it is not uncommon for that to occur

Unfortunately you cannot prevent food from getting under your partial denture.

Depending on how the partial is made and what teeth it is replacing, sometimes there is very little that can be done to completely avoid food getting under the partial. Making sure it fits as well as possible, meaning it fits the tissue well and fits in place with minimal movement will reduce the opportunity for food to get under it. If it doesn't fit the tissue well, having it relined could help significantly. Unfortunately, because the tissues of the mouth have a certain amount of give to them, it is nearly impossible to completely prevent food from getting under a partial. If it were to fit tight enough to completely prevent food entering, it would most likely result in the partial creating sore areas. Frequent rinsing will help dislodge food from under the partial if it continues to be an irritation.