Assuming you have a letter system, the best place to start I find is this video from LongBeachCityCollege study seminar:
www.youtube.com/watch?v=Ulk9BCRA83Y
at 22:39 the presenter introduces acrostics, which are sentences where the first letter or prominent sound of each word gives the letter pattern to recall. Common example is the lines on the staff of the treble clef in music: EGBDF "Every Good Boy Deserves Fudge" -- or the more modern "Empty Garbage Before Dad Flips"; or the natural notes of the strings on an acoustic guitar: EADGBE "Elephants And Donkeys Grow Big Ears" -- or for an old hippie "Every Acid Dealer Gets Busted Eventually". at 27:44 the presenter continues to mental pictures and the desired qualities for the best mental pictures -- He presents "SPACE", where S refers to Stupid or Silly, P refers to a play on words where a rhyme or sound-alike can give an easier to remember picture, A refers to Active where a motion or effect is attached to the image, C refers to clear in that the mental picture is vivid and not confusing, and E refers to Easy to see, where the picture is not too crowded with nonessential detali.
I'm still new at this, but here is an example. I am retaining the memory in letter pairs, the theory of this I won't go into here. I've taken to memorize the cube as a series of pairs of pairs, so that the 8 corners usually have about two pairs of pairs, and the edges usually have about three such. This gives me a better chance to see how far along I am in constructing the retention image. Here is one reading to attempt to retain with this technique: (my letter is speffz-like, but I substitute 'Y' for 'X')
Corners: OB SL . CW (one corner is in place solved)
Edges: HB CV . TP NE . DU IK . OY
after working this all day (still being new) with help from my wife I get
Corners:
Olivia
Bakes
Some
Large .
Chocolate
Waffles
Edges:
Hash
Browns in a
Ceramic
Vase . the vase is loosely wrapped in
Toilet
Paper some of which flaps from a
North-
Easterly breeze . I take the wrapped vase and
DUnk it into an
Icy
Keg . that contains
OYsters.