When making a Shiny App, put the ui section in a script called ui.R and the server in server.R. As long as they're loaded and in the same directory they will run without having to type "shinyApp(ui, server)"
When you have a lot of lines of code to run simultaneously but highlighting is a drag, put all the code within {} brackets and just run the line with the end bracket, it will run all the code.
Put comments on your code, and if you use at least four pound signs #### R Script will recognize it as a chapter for the outline table of contents, making it so much easier to find your sections in long scripts.