Electricity markets are an important tool in ensuring efficient operation of the modern power systems. They enable market participants to maximize the benefits they can receive from trading, at the same time bringing welfare improvements to the market as a whole. However, the participants need to optimize their processes to avoid being outcompeted by other traders. Furthermore, the efficient operation of the market also depends on bodies organizing and regulating it, i.e. market operators, system operators and policy-makers. Often there are multiple objectives that the actors involved in electricity market operation have to pursue. In this Thesis, the topic of participation in the electricity market and its operation is viewed from multiple sides, i.e. decision-making support methods, algorithms and tools are proposed for both electricity market participants and policy-makers. The subject matters covered are motivated by academic interest, as well as practical necessities expressed by actors within the power industry in Latvia. Consequently, decision-making methods, algorithms and tools related to large-scale energy storage technologies (scheduling, sizing) and cascaded hydropower plants (scheduling, hydroelectric set selection), as well as heating demand forecasting, which is a prerequisite for efficient combined heat and power plant participation in an electricity market, are developed in this Thesis. On the other hand, for the sake of policy-makers, decision-support is realized in the form of modelling, assessment and recommendations in regards to the influence of large cogeneration plants on the electricity market and, subsequently, the options to change the support these plants are subjected to. A common feature of these topics is the aim to increase the efficiency of electricity market operation, albeit from different perspectives.