I would suggest that when you work, you target a nice HD screen resolution like 16:9, 1920 x 1080. You can set this on the Game window, by selecting 16:9 as the aspect ratio, where it normally says Free Aspect. This helps while your in the editor.
Next. When you are laying out your UI, you should take advantage of the Anchor system in Unity. This allows you to place GameObjects in percentage from the Anchor. This also handles the scaling of the GameObjects.
For example: If you have a button that is 10% from the left and 40% down from the top, and it is rectangular with a 2% area, when you shrink the resolution, it will scale that button accordingly, while preserving its place in the view area.
Here is the link to the Unity Manual:Designing UI for Multiple Resolutions