I try to rack the first beer off the yeast as close to pitching time as possible. Generally, while the new beer is chilling I will rack the old beer to a secondary fermenter. Then I just dump in the new wort and aerate.
When I do this I try to keep the first beer at 1.050 or less (1.040 is better), and the next beer is at least 1.080 or so. If I'm pitching right on the cake, then I try to make sure the original beer is a similar style to the beer going on top of it. (i.e., APA to IIPA, a small Dubbel to Quad, Mild to Barleywine, etc.)