A good way to get an idea of how to make your foreground stand out from the background is to research similar games. If you look at games such as Sonic and Megaman, the backgrounds of the levels all appear either relatively darker or lighter than the foregrounds. In some cases, the background is blurred slightly, but this effect can have a negative impact on your game - what is the point in going through all the trouble to make a wonderful background if all you are going to do is blur it to make your foreground appear more prominent?
Additionally, in these games, the foregrounds are comprised of colors that compliment their respective backgrounds, and make them pop out. For example, if you had a prominently blue-colored background, one way to make your foreground pop out is to design it using warmer colors such as orange and yellow, because these colors complement the cooler blue colors. Castlevania did this exceptionally well:
Also note that the colors of tiles in the foreground are considerably brighter than those in the background, as can also be seen in Megaman X:
