The objective of Wireless Underground Sensor Networks (WUSNs) is to establish an efficient wireless communication in the underground medium. A magnetic induction (MI)-based signal transmission scheme has been proposed to overcome the very harsh propagation conditions in WUSNs. Due to a much lower vulnerability to the environmental changes, the MI technique has been shown to improve the system performance in terms of achievable data rates and coverage compared to the traditional EM wave based transmission. Two different approaches are known from the literature: direct MI transmission and MI waveguides, where many resonant relay circuits are deployed in the latter between the two nodes to be connected. In this work, digital transmission schemes are investigated for MI-WUSNs employing these two approaches. The influence of transmission parameters like symbol duration and modulation scheme are studied and new methods for their optimization are proposed. In this context, significant gains can be achieved compared to the naive straightforward approaches.