Think Metric

mil | cem | met | kay | mig | gram | kilo | ton | leat | meg

Symbol	Unit	Word	Examples
mm	millimeter	mil	"I was a mil away from death."
cm	centimeter	cem	"We're making progress cem by cem." "The President mustn't budge a cem."
m	meter	met	"Visibility is down to a few mets."
km	kilometer	kay or klick	"Give someone a cem and they'll take a kay." "A journey of a thousand kays begins with a single step." "Good grief, you missed by a country kay!" "Go the extra kay." "Only mets away but kays apart."
mg	milligram	mig	"A raindrop weighs just 4 migs."
g	gram	gram	"A gram of prevention is worth a kilo of cure."
kg	kilogram	kilo or key	"In the soft drink market, Coke is the 500 kilo gorilla." "A gram of help is better than a kilo of preaching." "Sugar Ray Robinson is the greatest kilo-for-kilo boxer of all time."
Mg	megagram	ton	"The truth is gonna land on you like a ton of bricks."
ml	milliliter	mil or cc	"20 raindrops make a mil." "A mil is the volume of one cubic cem (1 cm3)." "A cc is the volume of one cubic cem (1 cm3)."
L	liter	leat	"A cube 10 cems on edge is one leat." "A mil of honey will catch more flies than a leat of vinegar." "I love milk - I drink leats of the stuff."
ML	megaliter	meg	"A cube 10 mets on edge is one meg." "An Olympic-size swimming pool is about 2.5 megs."

1. These shortened common words are for casual spoken usage and make sense when written for song lyrics, novels, and screenplays.  Regular text should still use correct metric units, like "5 km" and "20 ml". 
2. The word "mil" is overloaded.  Since a "mil" can be either a millimeter or a milliliter, the meaning needs to be determined from context.  A better word for millimeter would be "mim", but that ship has already sailed.  No one is ever going to say that a GLOCK 19 (G19 9×19mm) is a 9 mim. 
3. Depending on context, kays can refer to kilometers as in "It's a short hike of only 3 kays" or to km/h as in "There's a 40 kay speed limit near the school".
4. It's best to stay away from the lesser known in-between unit terms, like hectometer, centiliter, or decagram, for the same reason you don't use archaic words that fell out of use a century ago.  For example, it would be unnatural to say "3 centiliters" instead of "30 cc" or "30 ml".  Note that some unit terms that are generally obscure may in fact be commonly used in specific geographic regions, such as Germans saying "deka" to mean 10 g. 
5. Avoid the Latin gibberish of the extreme prefixes except in scientific domains.  For example, write "90,000 km" instead of "90 Mm".  Or even use exponents, such as "3 × 10²¹ m" instead of "3 Zm".  A zettameter is Greek to everyone (except the Greeks).  Seriously, exactly ZERO normal people are going to memorize even a quarter of this prefix gibberish:
   quetta (Q) 10³⁰, ronna (R) 10²⁷, yotta (Y) 10²⁴, zetta (Z) 10²¹, exa (E) 10¹⁸, peta (P) 10¹⁵, tera (T) 10¹², giga (G) 10⁹, mega (M) 10⁶, kilo (k) 10³, hecto (h) 10², deka (d) 10^-1, centi (c) 10^-2, milli (m) 10^-3, micro (μ) 10^-6, nano (n) 10^-9, pico (p) 10^-12, femto (f) 10^-15, atto (a) 10^-18, zepto (z) 10^-21, yocto (y) 10^-24, ronto (r) 10^-27, and quecto (q) 10^-30
6. Variety is the spice of life.  In some regions people may prefer "klick" over "kay" for kilometer.  Some may prefer "key" over "kilo" for kilogram.  Local language variations are totally fine. 

"Before you criticize someone, walk a kay in their shoes.  Then you're a kay away and you got their shoes." Johnny Metric Carson

"Your kayage may vary." U.S. EPA after metrication

"It's not the years, honey, it's the kayage." Correct canonical version of Indiana Jones

"Knowledge without kayage doesn't mean anything to me." Henry Rollins if he had been born in Australia

"Take your dream and start breaking in down into kaystones." A Pragmatic Person

"My path has not been determined.  I shall have more experiences and pass many more kaystones." Agnetha Fältskog in the future

"Life isn't a matter of kaystones, but of moments." Modern-day clone of Rose Kennedy

"Remember to celebrate kaystones as you prepare for the road ahead." Even more visionary Nelson Mandela

"Sure, you can mix the flour, baking soda, salt, shortening, and the whole nine meters, but why wouldn't you just pull out a box of Bisquick?" Fully-Enhanced Semi-Homemade chef Sandra Lee