making of the shoe
 
a simple tutorial on box-modeling - page 27
   
   
 
Select the central polygon...
 
 
 

and extrude it three times, the stricter, the more evident is the crease.

 
   
 

Here the result after meshsmooth modifier

 
   
 
1 - 2 - 3 - 4 - 5 - 6 - 7- 8 - 9 - 10 - 11 - 12 - 13 - 14 - 15 - 16 - 17 - 18 - 19
 
20 - 21- 22 - 23 - 24 - 25 - 26 - 27 - 28 - 29 - 30 - 31 - 32