I just completed my second push up challenge workout.  I was worried because this morning and did a chest workout and was pretty wasted from that.  The goal today was to do 5 sets as follows:

27
19
19
15
Maximum (but minimum of 25)

My results:

27
19
19
15
30

That's a total of 110 push ups!  I was amazed that I pulled it off.  What I did was sit down before hand and visualize myself doing all 5 sets at the number required.  Then, before each set, I visualized just that set hitting the correct number.  Such is the power of visualization.