Containers need at least one drainage hole. After you water your plants, they water needs to drain out so that the plant's roots can get plenty of air as well, if the roots sit in stagnant water they will rot and kill the plant.
So, once you've made sure you have a drainage hole out of the pot, you need to make sure that you don't lose soil as the water flows out.
Gravel may seem like a good choice, but it actually prevents water from draining, it will fill the soil with water and then drain slowly allowing the roots to sit in that water for much too long.
Styrofoam packing peanuts work well, they stack up leaving plenty of air pockets for water to escape through.
Large shards of broken pots can add adequate drainage as well, the water will move both through and around unsealed terra cotta and drain out quickly.
A tip, once you've chosen a drainage solution for your container, add a layer of mesh screen between the drainage and the soil, so that you can easily separate the soil and drainage at the end of the season.
Simple steps to be sure that your plants have adequate drainage and will stay healthy and happy through the season. I'm Mrs. Fixit and it's just that simple!