How-to: export an Illustrator drawing to Photoshop without the drawing changing size

I do a lot of drawings in Illustrator, working directly over Photoshop images, after which I export the AI work to the Photoshop file and flatten it. This problem comes and goes, and I haven't yet figured out what combination of settings in AI or PS will keep the original  and the Illustrator addition the same size. When I do the original Illustrator image, I use the Photoshop base art as the template, so why does the Illustrator section sometimes fit perfectly, and sometimes require five or so minutes of trial-and-error to size it correctly? Any help would be appreciated.