A way I use to keep veg pots moist is to sit them on top of suitable containers ( old washing up bowls, etc.) filled with small pebbles. When filling the veg pots with compost (don't use peat based)trail some strips of fabric through the drain holes at the bottom to use as wicks. I usualy leave the base of the pot 3 or 4 cm below the level of the pebbles. Fill the pebble containers with water and leave the plants to sort it out themselves. Esp.good for tomato plants. The general rule is to water the pebbles and feed the pots (with the plants in!). Growing plants this way,you don't need as much compost and the plants will send roots through the bottom of the pots to take up water.