There are two schools of thought regarding the practice of fanning out a ream of paper prior to inserting the pages into a printer. If you normally do not fan the papers and they are sticking together, try doing so to see if it helps. Hold the paper in one hand and use the other hand to fan the papers like you would the pages of a flipbook. This may help separate papers that are sticking together due to humidity or other factors

click to enlarge
However, if you routinely fan the papers yet they are still sticking together, the act of fanning them could possibly be the culprit. When you fan the papers, especially if you are in an arid environment, it could actually generate static between the papers. Just as static can cause a sock to cling to a pair of slacks in the laundry, staticky papers will be prone to sticking together. Fanning the papers can also cause pieces to become crooked or stick out of the ream, which may bring about a paper jam.
Photo Credit: sxc.hu/LotusHead