Nonstandard finite difference (NSFD) methods are known to offer numerous advantages over traditional techniques for numerically solving various problems in science and engineering. However, in general, they are only of first-order accuracy. In this chapter, we present four new classes of modified NSFD methods, which are not only elementary stable and preserve the positivity of solutions, but are also of second-order accuracy. We also present a series of numerical simulations that support the theoretical results and demonstrate the advantages of the new modified NSFD methods when compared to other classical standard and nonstandard numerical methods.