Configure Play Store to download temporary files to SD Card
Unfortunately, there is (currently) no way to do so on a non-rooted device, at least not to my knowledge.

The Play Store app always downloads to /data/local as far as I know. So if your device is rooted, you could create a directory wherever you like, and symlink it here. This could be done either from a terminal app, via a file explorer supporting the creation of symlinks (and root), or via adb shell. For the terminal/adb-shell variant it could look like that, assuming your sdcard being mounted at /sdcard:

$ su
# mkdir /sdcard/playstore-temp
# cp -r /data/local/* /sdcard/playstore-temp
# rm -rf /data/local
# ln -s /sdcard/playstore-temp /data/local


Explaining the commands:


get root access
create the new "temporary storage directory"
copy all current content (recursively) to the new location
remove the old location
create a symlink from the new location to "fake" the original


I didn't try this, but it should work. Like with Link2SD (which uses the same principle for installing apps on SDCards), for the Playstore app this should be "transparent", i.e. it should be fooled into thinking it still works the same way as before :)