Use the anchor behaviour on parts of your UI to have them fix to different parts of the screen. Along with the new scene graph tool (new pinning) I'm finding extremely easy to create UI layouts for different aspect ratios. Supporting a complex UI in both portrait AND landscape though will always be quite painful so you should probably stick to one or the other if your UI is complex..
Once your anchors are in you can then add further refinement based of the current aspect ratio. For example you might want to completely reposition buttons if the phone is very narrow.
When testing it you can resize your preview window in realtime to see what it will look like at different resolutions. You should make sure your game looks good between aspects of 4:3 (1.5)ipad all the way up to 2.3 which is the really thin Sony Xperia display. Most common aspect is 16:9 (or 1.77) which you should probably use as your default design view if you are developing for mobile.