Bacopa
Sutera cordata

Description:
A vigorous, trailing grower with white blooms all season.  Use in hanging baskets, combinations and window boxes. 

To maintain constant bloom you will either need to monitor the plant closely and water before the soil gets too dry or you could plant with a plant that does wilt. Some good plants to consider are verbena, coleus, petunias etc. These companion plants will serve as indicator plants to let you know when the plant is dry.