it's all downhill
1. Something is steadily worsening. Once my parents open my report card and see how bad my grades are, it's all downhill from there.
2. Something is increasingly easy (as if literally traveling downhill, as opposed to uphill). I'm sure I can finish the rest of the experiment on my own—it's all downhill from here.
it's all downhill
Also, it's all uphill from here. See under downhill all the way.