The problem of single application development that can work in widely used modern mobile platforms (Android and iOS) is dicussed. Current situation in building of crossplatform applicatioons is studied. The choise of appropriate development tools has been explained. The basic principles and rules of design and development of crossplatform mobile applications using chosen Xamarin.Forms technology has been described. The paper consists of two parts. The first part describes purposes and benefits of used Xamarin.Forms crossplatform technology and contains technical requirements. The Xamarin.Forms technology using with C# object oriented programming language. The second part describes the best practices of using this technology in current project: MVVM pattern definition for devlopement using best style OOP; C# asynchoronous programming for creating comfortable and fast for use application; custom controls creating used in current project for best UI experience; using platformspecific code with DependencyService; customization of standard controls with Renderers; final application optimization to reach maximum performance and minimum battery consumption at a time (results of battery time optimization are presented). Finally, studied and written about using of new features of Xamarin.Forms by big developers’ community. Examples of software code and application screenshots used in application are given. The work shows the stages of the development of the mobile business application modules, which is already used in commercial product; all of the given examples are thoroughly tested during the development process and in real work, that allowed to make conclusions about best practices. The use of the developed sowftware allowed increasing the efficiency of trade accounting due to decreasing of monotonous operations quantity and as a result, the decreasing of errors in staff work, that already gave opportunity for money economy.