Maintain even soil moisture. Mulches conserve moisture and moderate soil moisture extremes. Even with mulches, weekly irrigation will be necessary if there is no rain. Fluctuations in soil moisture often leads to an increase in blossom-end rot. Make sure tomatoes are planted in an area with good drainage.
Excess water causes roots to smother leading to reduced calcium uptake.
Do not hoe too deeply. Deep hoeing can damage roots and thereby reduce calcium uptake.