2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1
Add cable tension and possibly, but not likely, adjust the high limit screw. Since you say rear derailler, I'm assuming you mean largest cog, and not largest chainring (which would be on the crank where your pedals are attached). But, in either case, more cable tension would do it. (I don't want to confuse you but the only way more cable tension would not be the fix is if you had a reverse action rear derailler).
Go to www.parktool.com to read a good write up on derailler adjustments. The advice is fantastic in my opinion.
First, shift the chain to the smallest cog and check to see if there is slack in the cable at the derailleur. If there is, turn the barrel adjuster all the way in (clockwise, looking from the cable side) then back it out one full turn. Next, loosen the cable clamp on the derailleur and remove the slack. After this is done, proceed with cable tension adjustments. bk