There's no one-size-fits-all answer to this question, as the best way to slow down the growth of your grass will vary depending on the type of grass you have and the climate you live in. However, there are a few general tips that can help:
1. Mow your grass higher. This may seem counterintuitive, but taller grass actually grows slower than shorter grass. That's because longer blades of grass produce more shade, which inhibits the growth of new shoots. So raise your mower blade and cut your grass a bit longer than usual.
2. Fertilize less often. Giving your lawn too much fertilizer can actually stimulate excessive growth. If you're using a chemical fertilizer, apply it at half the recommended rate. And if you're using organic compost, make sure it is fully decomposed before spreading it on your lawn (compost that hasn't finished decomposing can release high levels of nitrogen, which can promote rapid growth).
3. Water deeply but infrequently. Frequent watering encourages shallow root systems and weakensgrass plants, making them more susceptible to drought and disease. Watering deeply but less often encourages deeper roots and stronger plants. Aim for 1-2 inches of water per week (apply it all at once so that it penetrates deep into the soil).