IfeTahu (click it to refer to his post at Art of Problem Solving), a IMO 2014 participant from US team, has recommended the following MO books:"101 Problems in Algebra", "102 Combinatorial Problems", "104 Number Theory Problems" by Titu Andreescu and Zuming Feng
"Number Theory - Structures, Examples and Problems" by Andreescu and Dorin Andrica
"Geometry Revisited" by H.S.M. Coxeter and S.L. Greitzer
"Plane Euclidean Geometry - Theory and Problems" by Christopher Bradley
"A Course in Combinatorics" by Wilson and Van Lint
"Enumerative Combinatorics" by Stanley
"Graph Theory" by Diestel covers difficult combinatorics for IMO level
"The Probabilistic Method" by Alon and Spencer "MOP 2006 Homework Problems" - This handout was used to train the US IMO 2006 Team, and it contains a lot of extremely difficult problems in algebra, number theory, geometry and combinatorics.
Some of the above books may be available on the Internet. Go google them.
Another way to improve your MO skills is to regularly visit Art of Problem Solving (AoPS, Lim Jeck's favorite website. He is "oneplusone" at AoPS ) to solve and post MO problems.