If you go to your user preferences and go to Theme, you will be able to customize how selected objects are show in your viewport. Two key values that can help with this are the Outline Width and the Active Object color.
Additionally, I find it easier to work against a dark background and use the Solid OpenGL lights to accentuate my objects against the background. Also it's not uncommon to find it hard to pick out objects that are close to each other at times, an easy way to tell this is to just change the viewport shading to Wireframe with Z.
The default theme has its shortcomings so my personal setup of many years is a bright diffuse lamp from the top left followed with some subtle darker colors to illuminate the other corners. I have a thick white outline to show my selected objects in both object and edit and a dark gradient background. You could also just use one of the default themes under Presets in the same theme tab.