If you'll run some searches you will find there have been many threads addressing this problem.
The best approach is to flush out the shifters with WD40. The factory grease gets hard with time and there may also be a build up of dirt and other crud in the mechanism.
If that fails you can try new shift cables and cable housings, especially the short housing just before the rear derailleur. Shift housing wear and often are corroded and/or dirty causing excess friction.

Al