Antonyms for the word "be wasteful" are to be thrifty, economical, frugal, and prudent. Being thrifty means to avoid waste and to use resources efficiently. It is a crucial skill that can help us save money and protect the environment. Being economical means to use resources wisely, without wasting them. It involves making careful choices about what we buy and how we use it. Similarly, being frugal means to be sparing or economical with resources, thereby avoiding waste. And, finally, being prudent means having good judgement and taking care not to waste resources, but instead making the most of what we have.