Tons o' hay. Nice and long and stalky so she has to really put effort into chewing it.
When Sorrel had molar spurs, I cut her pellets down to about 12 a day, plus leafy veg only, and lots of hay and dried herbs. Her dentals went from roughly every 6 weeks to every 12 weeks.
Plantain is very high in fibre I believe, as is echinacea (coneflower), you can get both of these in dried form.