It's all about the foundations
I've always suffered from memory problems, especially when a subject isn't particularly interesting from a spiritual point of view - so for my work stuff (network & firewalls) I always had to delve deep into the understanding of how it all hangs together.
This makes initially learning a 'concept' longer, such as a new area of technology that I haven't worked on before, but once you do this you have created a framework within your mind upon which to hang 'information'.
After a time not working on a particular type of device, I can forget a lot of the details, but the infrastructure (i.e. the fundamental understanding) is still there so it can be picked up again in no time at all.
After a few years of this I discovered that the same infrastructure can be employed to learn new kit in a very short time, even under lots of pressure, a very useful skill to have as a consultant with a new job every 6 months or so.
It came as a complete shock to me a few years back when a colleague in a position of trust for a major company, running their firewall estate, didn't know about subnet masks. He said 'that's just the routing stuff' - presumably catagorised under 'someone else's problem' in his head.
He was surprised to learn that his firewalls were routers. Apparently he went on a firewall course to become a firewall engineer - not moving up through the network support path that I thought was ubiquitous - he didn't even know what the 7 layer model was, let alone what they were.
Mind you, I forgot the name of the interface scanning command* on splat in an interview, even though I could recall all the flags to set - still got the job though :) I guess bullshit is a skill too - as long as you can back it up.
*yes, tcpdump