How do I force iPhoto to download new photos in my Photo Stream?
I have found a better way. One that doesn't involve resync'ing the entire stream.


Quit iPhoto
Open up Activity Monitor
In the search field enter: photostreamagent
Select all the PhotoStreamAgent processes listed
Click the Quit Process button
In the confirmation dialog that pops up click the Quit button
Confirm that all the PhotoStreamAgent processes are no longer alive by waiting for the Activity Monitor window to show no processes with that name
Re-start iPhoto


When you restart iPhoto it will re-start the PhotoStreamAgent process for your iPhoto session and kick off a new Photo Stream sync. You'll get your lastest pictures updated and you won't have to sit through a full re-sync of all your Photo Stream.



Update for Yosemite where the process name has changed to iCloud Photos. This is the what you want to kill:



Update for Yosemite 10.10.4 where the process name has changed to Photos Agent. This what you want to kill:



You can also do this, quickly, from the command line like so:

sudo pkill cloudphotosd
sudo killall cloudphotosd