It’s very simple: record the number of times that a batter moved a runner over who will eventually score, but that he did not get credit for an RBI. The leader in 2008 is: Justin Morneau, with 73 Batting Assists. (Can you guys think of a better name?) There were 65 runners that he moved into scoring position (by hit or out) that scored in a subsequent at bat. And there were 8 runners that scored while he was batting (by out), of which he did not get an RBI.
How about if a batter does NOT advance a runner even one base (or worse, gets him doubled off)? The MLB leader in 2008 was: Jeff Francoeur with 319 runners who were blocked. I’ll call these Batting Blocks.
Finally, the leaders in the ratio between Batting Assists to Batting Blocks is: Joe Mauer. The league average ratio is 1 assist per 5 blocks. Mauer had 69 assists and 170 blocks. In "Linear Weights speak", he was +29, followed by Ichiro and Carlos Guillen at +22. On the bottom side, we have Francoeur and Corey Hart at -21.
Since 1993: the leader is Barry Bonds at +258 and Derek Jeter at +231. At the bottom of the pile is Tony Batista at -128.