One way to go is to render on screen controls for mobile/touch devices. However, I feel like this isn't the best approach. In order to have the best possible experience for your game it should respond to touch events within the game itself. (Touch a unit to select it, touch & drag an item from slot to slot, etc)
Take Plants vs Zombies on iPad for example. One of the reasons why it's such a great game on that platform is that the gameplay itself meshes well with touch controls.
On screen controls are definitely the "easy way out" when trying to port your game to mobile touch environments but they feel clunky and probably won't give your users the best experience with your game.
Some games just aren't meant for touch interfaces and if a developer wants to support mobile/touch they're better off designing their game to accept that style of input from the start.