Is it possible to permanently improve long-term memory? Many similar questions here ask either about working or short-term memory, or about various tricks and techniques to efficiently remember information. My question is, is it possible to improve the capability for remembering long-term itself? So that learning/memorizing something would take less time or repetition, and the acquired knowledge/information will be stored for longer and with less distortions.

The effect might not be permanent, but it should be prolonged and substantial. To illustrate my point, let's assume for argument's sake that learning poems has the effect of improving long-term memory. This would mean that, after regularly memorizing new poems for several weeks, i find that i'm able to memorize new poems spending less time, the poems retain remembered for longer. I'm also able to remember content of non-fiction and fiction texts better too, so that i can recall tricky facts and logical inferences, which before memorizing poems i tended to forget. So, is there a method to improve the general efficiency of your long-term memory? In other words, ability to retain more information for longer periods with less effort?
This is what I do to store something in the long term memory. 

1) Place it in the short term memory (those methods depend on the material).


Use mnemonic memory techniques.
Repeat the facts in different order.
Relate the facts to something that you already know.
Organize the facts in a logical order (order improves memory).
Try to find semantic meaning the facts.
Practice with active learning methods like writing and talking rather than with passive learning methods like listening and seeing.
Make the learning experience engaging. Get involved with different activities that include using the material you have learnt. For example, teach it to others.
Draw a vivid mind map of what you learnt.
Jot down self questions on the material you had to remember.


2) Take a 10 minute break.
Answer the test that you designed and the mind map that you drew in step 1, all from memory.
Repeat step 1 for the facts which are the answers to the questions that you didn't answer correctly in step 1.

3) The day after. Do another repetition.
Repeat what you didn't in step 2. This time mark the questions to your test that weren't answered correctly.
During the month, every day, try to answer the questions that were marked as wrong by the previous day.
So for example, if in day 3 you didn't answer right to question 4,6,8.. mark them and repeat only them on day 4. If you answered question question 4 and 8 correctly and 6 incorrectly, mark question 6 as wrong. Repeat the process every next day, until all the questions are right, then stop the process.

4) The next month from the day you started step 3.
Repeat step 1-3. First, answer all the questions in your test, then make a general repetition. Design a new test..answer with the same technique described in step 3 to eliminate memory fading after the revision.

5) After half a year repeat step 4.

Just to add. I memorize dictionaries using this technique. If you have a lot of meaningless data to memories (dictionaries, encyclopedia, lexicons, fact books), you need a technique like mine, because most of the other techniques out there are too passive.

My technique also trains the memory better. It helps you to transfer memory from short term to long term (Especially the mnemonic techniques). The example that you bring relates to the idea of familiarity and semantic meaning. If you memorize English words, overtime it would be easier to memorize more English words than, let's say, french words.
It does only increase long term memory for a specific topic, not for everything.

Other factors that help to improve your long term memory are amount of sleep, physical health and nutrition.

References:


Tony Buzon. Memory books.
Introduction to psychology book  (A-level). Cognitive psychology chapter.
Self-study skills gained through university.


Semantic memory might be the reason you find it easier to remember more and more poems as you continue reading them.

Some research:
Semantic memory - http://www.sciencedirect.com/science/article/pii/S0022537173800568