A few suggestions:

1. Max out the dictionary size option on the compression tool you use. This allows more internal re-use of common sequences found in the data. It can waste space on a few small text documents, but with many big photos, the dictionary size is relatively small compared to the data contained, and will pay for itself quickly.

2. Compress each scene/batch in a different archive. This relates to #1; allowing the parts of the 16x16 block data that appear frequently to be replaced by a dictionary entry. If all the photos are about the same, it can really squeeze the redundancy out much more effectively than if you have a group of say vacation pics with food, people, sunsets, landscapes, and lots of other random subjects.

3. Like 2, compress jpeg and raw files separately. This increases redundancy.

4. Try several different compression engines. Don't assume zip or rar or z7 will work the best on everything. If you use command-line tools, it's pretty simple to just use all of them on each batch, all you have to do is wait and delete the largest archives of the grouping.

5. Have reasonable expectations. I doubt you'll do better than to half the footprint, and even that could be a tall order depending on the camera, image, and algo. Many raw files these days are already compressed in-camera, so there's less overhead to wring out.

6. Experimental. Try using a batch tool to combine many images into a large image file, then try different compression methods designed for images on that file. As a bonus, this should make non-image compression more efficient as well, but it might be a lot of manual work to save just a little bit; something to consider if you're long on time and low on cash/space.