First, be sure to store them in your crisper drawer in your fridge, and try not to store them for too long before cooking. After you wash your green beans, be sure to dry them well. Moisture will make it difficult for the beans to get crispy. You can also use your oven's broiler for about 2 minutes at the end of cooking to really crisp them up--just be aware that they can burn easily this way, so keep an eye on them.