Storeing Jerry Cans

i just put them away never done anything to them except keep them sealed never had a drama. and they were my dads jerrys so they are fairly old ones.

Do nothing, after having diesel in them they will never rust internally, clean the the filler neck and seal, paint any bare rub spots on the outside and store them preferably on a shelf, never on a dirt or concrete floor and they will keep for the reat of your life.
I've got some that are over 40 years old and in perfect condition.
If using plastic ones they would be best stored in a cool place like under the house or in the corner of the garage, the weather will eventually get them other wise.

Maintaining jet aircraft which use AVTUR- similar to kero/diesel, we come across this issue with fuel tanks during aircraft storage.
Its a fact that water will settle to the bottom of your jerry can and if the paint is broken, will cause corrosion, and if not will still lead to algae growth. So you are spot on with your concerns.
With fuel tanks which hold 80tonnes of fuel it is an issue and Boeing stipulates that fuel tanks must be filled to the brim to reduce the air/moisture percentage in the tank, and an anti microbial solution is to be added.
Even when this is done, periodical water drains must be carried out to drain the settled suspended water in the fuel....
But i doubt you have to go to anywhere near that extent for a jerry can! Your concerns are on the right track, but i really wouldnt worry too much mate.
I definitely wouldn't leave some diesel in the bottom which will go stale, allow water to settle and algae to grow and i wouldnt flush them out with water. Just drain well, make sure the lid seal is in good condition, close tightly and store off the ground (as recommended above).
Cheers!

As above.
And I try to use them at least every few months.
When filling up I might take one or two Jerries and fill them up as well, then top the tank up later.
Reckon the sloshing the fuel around the inside keeps the rust at bay.

Can anyone advise where to get new seals for metal jerry cans? I've got two that are 30 years old and in good condition but the rubber seals now leak.

C'mon! is this a trick question??