The humidity is often so low - we were at 4% yesterday - that soil will dry out within hours. You can mitigate some of this by adding an organic mulch around your plants - anything from compost to bark will help keep the soil moist, and the worms etc will go to work breaking it down, further helping improve the soil.

You'll have much stronger plants if you can water deeply and more infrequently - easier said than done with new annuals with tiny root structures, or purchased plants that come root-bound in tiny containers.

Mulch will definitely help. I usually use a variety of materials for mulch. Basically whatever I can get for free or low cost which right now is wood chips and brewery grains.. I also grow my own mulch with alfalfa, clover and whatever veges are going to seed.

Deep watering is great but also with sandy soils I have found more frequent lighter watering works well also. I prefer to dig with a sharp shooter shovel or hand trowel now and then to see what the moisture levels are.