Updated: Oct 2
Cacti are drought-tolerant plants. That means they can endure a little bit of dryness but not too much watering. So, to keep a Cactus thriving, it is vital to control watering. Here is a method that works great: find a roommate for your Cacti, especially for Cacti growing in pots.
By talking about a roommate, I mean other succulents: also drought-tolerant, hardy, and can take a little more water. For example, I plant some Sedum morganianum (Donkey Tail) and Haworthia in the pot with my Pachycereus schottii (Totem Pole Cactus). Those succulents can hold and consume more water, and they do not take too much space. When established, Sedum morganianum tends to grow into long strings, which makes them great decorations. That is also why you can find them in many succulents designs.
Generally speaking, succulents that are hardy and faster-growing are a great choice as roommates for Cacti. In case you overwatered your Cacti, those roommates can help take the extra water and prevent prolonged wetness of the soil.
Curio rowleyanus, syn. Senecio rowleyanus (String-of-pearls or String-of-beads) and Senecio Herreianus (String-of-watermelon) are also good choices.