Breaking In

Most cubes when new, are difficult to twist, as there might exist some excess friction in between them, caused by miniature bumps on the plastic. These need to be ironed out so that you can use your cube to its full potential. There are many ways of breaking into a cube, like sanding your cube, or using Vaseline over time.
Sanding, I don't recommend, as you may end up damaging your cube more than required.

Vaseline on the other hand, being petroleum jelly, actually corrodes these unwanted bumps over time. If you solve the cube regularly each day for up to 2 weeks, your cube should be broken into well. This period of 2 weeks depends on the type of cube, you're using, for the regular store bought Rubik's brand, the 2 weeks are fine. For others, it may vary a little.

Once you're done breaking into your cube you should clean it up and use a different lubricant, like Silicone spray or Silicone oil. I'll get to the lubrication bit soon. For those of you who're not too familiar with Rubik's cubes, please see my Parts and Disassembly page above or click on the following link: http://cubepedia.blogspot.in/p/parts-and-disassembly.html

Until next time, happy cubing!