I'm unaware of any formal principles of storage but a few tips I've gleaned from friends and experience:

Reducing what you have:
If you don't use something in (insert appropriate time limit here) you can likely get along without it.

Pension off unneeded goods, put them up for sale on E-bay or Graig's List. Alternatively take advantage of tax credits by donating to charities and charity organizations. Or find sympathetic people who are just starting out in your areas of interest.

Work with neighbors to share equipment. Does everyone need a lawnmower? Or could you just slip a neighbor with a lawnmower a few dollars to use theirs?

Get familiar with local rental shops and what they carry.

Packing stuff away:
Favor soft-sided luggage and storage containers. They can be stuffed into tighter and odd-shaped spots. And when the contents of a container are in use a duffel bag or sack folds up. Hard cases take up the same space full or empty.

Be careful with vacuum bags. They can save a lot of space by compressing towels, pillows, and sweaters. But if they leak they can destroy shelf systems and containers. Compression sacks, which use banding to remove air, are less likely to decompress at inconvenient times.