Designing the UI
The UI of a game should be designed while taking two aspects into consideration: what the look and feel of the game is and what are the actions the main character will perform, and how.
Begin by defining a list of functions that must be included in the UI, things such as score, available lives, real and virtual money amount, cross-hair, and mini-map, whatever is absolutely necessary to be displayed in the interface of the game.
Then map these fundamental functions to different interface methods. Take each item on the list and ask yourself which is the interface method that better fits the needs of the game with regard to that specific interface component. For example, assume we need to show the health status of units in a strategic title: will the player better benefit from a spatial method, such as drawing a health bar on the top of the unit, or a diegetic method, such as displaying the health status as actual damage to the unit model/sprite?
When making such a decision, always...