Best way to save ink in inkjet printer when rarely used?

All I can suggest is to print a test page on the weeks you don't use it to keep the nozzles clean.

The ink in the cartridge does not dry up, usually the nozzles get clogged with dried ink from non use.

Other than that go with a color laser printer.

Here is an article from HP on the subject.

EDIT HP article is no longer available, but there is an Internet archive of the original.

Also it has similar advice in this document (scroll down to step 3)

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Additionally to Moab's answer, I suggest to not unplug the printer's power cable.
Most devices will do a self-test, wasting lots of ink, when the power was interrupted.
The cost of the power consumption in stand-by mode, is nothing compared to the cost of the ink that's wasted during avoidable self-tests.

I read this some time ago in the German "c't" magazine.

Modern inkjets have become better about clogging. It will depend on the model, environment etc, but in general a modern inkjet can happily sit for a week or two without clogging (I also have a rarely-used printer).

In principle you could take out the cartridges/print heads when not in use and put them into a vacuum airtight bag/box or similar; that should prevent clogging. However, this is probably much more work than just printing a page every two to three weeks, so this seems the best way to go.

You can just set yourself a reminder to print a test page every start of the month or so. No need to even hook up a computer - most printers will print a test page if you hold the "paper feed" button when switching it on (or some similar combination, check manual).

Doing this once a month should generally be enough.

If you really encounter clogging:

Light clogging is usually easy to remove by wiping the nozzles with alcohol. I've used that to revive a printer that had sat unused for almost a year. Also see e.g.

How to revive an inkjet printer — partially clogged/dried up?