What in the brain/body or the Universe around it helps determine which memories to retain long-term and which to let 'evaporate' with time?

The aim of this question is practical : I am learning languages etc. and want to learn tricks to help me remember learned elements much longer and reliably.

As someone with their ear to the ground about this sort of thing, and an ex-ESL teacher who’s read what’s regarded as the best research, I am aware of some language-specific and non-language-specific factors that can help.

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***It appears that…

(a) ...except in maybe autistic persons, at a certain stage of development the brain naturally starts to discard detail and generalise;
AND
(b) ...the brain operates as it does, letting most experience-data fade, in the interests of operating efficiency : though the brain has the storage capacity to store EVERYTHING we experience in our lives, accessing what we want in all that data would be prohibitively slow if we went this route (so the recent theory goes.)

(One theory also contends this is why normal adult brains generalise instead of being open to every iota of detail like Asberger’s / autistic people who can remember and reproduce unusual amounts of data, Rain Man-style.)

***The brain stores in long-term memory things perceived to be related personally to one’s self because the part of the brain that deals with “personal stuff” also deals with long-term memories.

***Memories can be associated with and triggered by sense stimuli esp. I think smell.

Scientists have discovered our favourite dishes are not determined so much by taste but rather associations with positive events i.e. when we were feeling happy etc.

***Attaching a story or what I would call an other “meaningful” mnemonic hook interconnecting elements of data to be memorized is how the world memory champs do their thing.

***Finally, these are just my pet theories, but I think some memories must be written more indelibly in one’s brain/body neural system if…

(a) ...they are perceived as high-priority “important”
&/or
(b) ...they are associated with high-priority “important” events, stimuli etc.

For example, unusually “positive” and “negative” experiences seem to be held on longer.

A strong emotion associated with some experience-data
seems to ensure
it will not necessarily always be in the forefront of operating memory,
but can be recalled, whether one wishes to or no,
years down the track.

One wonders if this evolved in us and was retained
because it helped our forerunners survive by :

(i) remembering and thus avoiding
“conflict” or “danger” events or situations
associated with high feelings etc. of
anger, fear, hate, stress etc.
(consider maybe Vietnam vet flashbacks of traumatic events);
...as well as, perhaps…
(ii) remembering and thus seeking

actions or situations
associated with high feelings etc. of
“comfort”, love, pleasure, relaxation, “safety” and “security” –
a sense of “wellbeing”
(think, say, of your most pleasurable memories
of time spent with partner, children, friends and family
in pleasant activities).

Note that my theories (a) and (b) above may be related to the principles of perceived personal connection or relevance and senses e.g. smell being associated with and triggering a memory, which suggests they are correct.

FEEDBACK FROM PEOPLE
WHO ARE EXPERIENCED + TRAINED IN THIS AREA,
OR
WHO CAN REFER ME TO SOUND RESEARCH / LOGICAL ARGUMENTS,
WOULD BE PARTICULARLY APPRECIATED!!!

THANKS.