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

None seem to be known. I have not observed any biases either. (I'm Level 4, so I've hacked a few …)

"Just hack more often" might not be the answer you're looking for, but …

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.