you might want to check out hall of famer steve carlton's web site. just google him to find it. he has some great pictues and explainations of how he threw his slider (which was considered one of the best in the history of the game).
remember you work on building arm strength and sound mechanics (everyone has their own personal style within some fundamental parameters of good mechanics. that is why the great pitchers that throw for years without major arm problems and get people out do not all look the same).
once you develop a solid base of reasonable arm strength and good mechanics, usually you throw one of three types of bullpens: 1) velocity-you focus on throwing harder; 2) stuff-you focus on gaining maximum movement and rotation on your pitches; and 3), location-you focus on locating your pitches. it is difficult to work on all three of these at the same time, especially in the off-season.
hopefully you are working on gaining some strength in your body and core while you are in the off-season. strength usually equates to better velocity and durability. this is normally gained in off-season and pre-season.
probably more than you wanted to hear.