Is there a way to increase the odds of getting a portal key?

I haven't done any systematic data collection on this, but when a portal is near death (via enemy attack or natural decay) i sure seem to farm portal keys with much more frequency. Anyone else experienced this?

Discounting "hack more" as an answer, there is one technique you can use - destroy links. Some percentage of the time (I'm not sure how much), destroying a link will result in a portal key being dropped near one of the portals. If you want keys for a specific portal, try destroying things that link to it.

With version 1.33 (or 1.34 I am not sure about the exact version number), you will get a portal key (about 80% or something like that) if you do not have the key of the portal already. If you own at least one (or more) keys of that portal, then percentage of obtaining key with hacking is really low.

If you already have some keys in your inventory; you can drop them on the ground and take them back after you hack the portal. By that means, you can obtain a lot of key belong to that portal.

All other answers are for the old versions of Ingress. You have to do Glyph hacking and use the request more command, then you will get 1 to 3 keys in 90 % of hacking for a right guessed glyph sequence. This applies for the current version at the time of writing, 2020-11-15.