Systems thinking is about getting away from oversights resulting from the over-specialization of most current professionals, especially scientists. It can be formally hosed oil systems theory, of a selected type: there are many around, or informal. The concept of informal systems thinking is important because there is a lock of education in systems thinking / systems theory, there is a lot of unavoidable narrow specializations, and hence a lack of consideration of holism. Successful decision makers in business and government concluded that their decision analysis training together with practical experience help their informal decision-making. The keynote paper deals with the prescriptive orientation to decision-making: instead of regarding people as perfect rational individuals, we developed a frame procedure for multi-criteria decision-making to complement intuition and to master interdisciplinary cooperation on formal and informal principles. We discuss how decision makers can integrate analytic results into their informal requisitely holistic thinking, without disrupting it, and how training in formal decision analysis call support informal decisions. This paper introduces how informal systems thinking way incorporated in solving several real-life complex problems with contemporary operations research methods, presented by practical applications oil the micro level (information system's development, creditworthiness assessment, environmentally oriented business decision-making, process benchmarking, choice of investments) and on the macro level (measuring the globalization of national economies), as well.