Best Way To Superimpose Part of Picture A into Picture B?

I want to add a body shot of myself in photo A to a tropcial beach photo B.

Right now my workflow is as follows:

1) Drag source photo A and target photo B onto desktop
2) Open both with Preview
3) Work with Preview adjust color to try and match light/color in source Photo A to match that of target Photo B
4) Do a gross block select (Preview only allows rectangles) of the part of image in Photo A that is going into Photo B
5) Paste selected part of Photo A into Photo B
6) Resize/position the pasted area so the "scale" and angles are correct
7) Save the new Photo B alongside the old Photo B
8) Cut and paste as much as possible from old Photo B into new Photo B to minimize "edges" (using medium rectangles)
9) Cut and paste further within new Photo B to try and minimize "edges" even further (using small rectangles)
10) Import new Photo B into Aperture and use brushes to complete edge minimization

This is slow and tedious.
Is there a better way to do the above?
Is there a way to "lasso" a part of one photo and paste it into another (free hand)?
Or a way to do a "green screen" approach?

Any suggestions would be greatly appreciated.