Use a very subtle stroke or border on the transition between two background colours.
If you're using two background colours that don't have very good contrast against each other, and for whatever reason don't want to change one of them, you can make the transition point "sharper" by adding a very subtle stroke or border between the two background sections. Most people won't notice that the stroke is there unless they look for it, but it'll make the transition appear much cleaner.
In the example below you can see the top transition has no stroke between them, and one has been added to the bottom transition.
This stroke can be far more subtle that you might expect, and still give the effect you're after.
