If you want to memorize long lists, large sequences of numbers built up by hundreds of digits and ultimately recall the calendar of 20th century, then all you need to do is learn the Dominic System. Its based upon mnemonics and their effectiveness and power is fantastic.
Dominic O'Brian come up with this system and it is explained thoroughly in "How to Develop a Perfect Memory" book. Also you can find smaller article that stick strictly to the point:
here. Be aware: this requires practice and you won't gain overnight enough experience nor understanding to utilize this system up to its fullest.
As for improving short-term memory I'd recommend doing the following with persistence:
Multiply 2 digit numbers
in your memory. Don't write them down. Visualize the whole process as if you'd multiply them on your paper. Make both numbers 2 digit ones. Start with easier ones like 11*12 until you get the hang of it. Then as your skill increases make them harder, something like 69*84, 49*73, etc.
Do these three times a day; 3 multiplications per day, that's enough. After a few weeks you will be amazed how "easier" the process will go and that you will be able to recall larger sequence of numbers without relying on mnemonics and just using your memory.
Yes, exercising your memory is like working out and stressing your muscles. No pain, no gain.
PS: Also, if you are studying then I'd advice endeavoring into
mind maps, too.