There are two main methods of describing concentrations: by weight, and
by molecular count. Weights are in grams, molecular counts in moles. (If you
really want to know, a mole is 6.022*10^23 molecules.) In both cases, the
unit is usually modified by milli- or micro- or other prefix, and is always
"per" some volume, often a liter.

This means that the conversion factor depends on the molecular weight of the
substance in question.

mmol/l is millimoles/liter, and is the world standard unit for measuring
glucose in blood. Specifically, it is the designated SI (Systeme
International) unit. "World standard"is not universal; not only the 
US but a number of other countries use mg/dl. A mole is about 6*10^23 
molecules;
if you want more detail, take a chemistry course.

mg/dl (milligrams/deciliter) is the traditional unit for measuring bG (blood
glucose). All scientific journals are moving quickly toward using mmol/L
exclusively. mg/dl won't disappear soon, and some journals now use mmol/L as
the primary unit but quote mg/dl in parentheses, reflecting the large base of
health care providers and researchers (not to mention patients) who are
already familiar with mg/dl.

Since m.h.d is an international newsgroup, it's polite to quote both figures
when you can. Most discussions take place using mg/dl, and no one really
expects you to pull out your calculator to compose your article. However, if
you don't quote both units, it's inevitable that many readers will have to
pull out their calculators to read it.

Many meters now have a switch that allows you to change between units.
Sometimes it's a physical switch, and sometimes it's an option that you can
set.

To convert mmol/l of glucose to mg/dl, multiply by 18.

To convert mg/dl of glucose to mmol/l, divide by 18 or multiply by 0.055.

These factors are specific for glucose, because they depend on the mass
of one molecule (the molecular weight). The conversion factors are
different for other substances (see below).

And remember that reflectance meters have a some error margin due to
both intrinsic limitations and environmental factors, and that plasma
readings are 15% higher than whole blood (as of 2002 most meters are
calibrated to give plasma readings, thus matching lab readings, but this
is a recent development), and that capillary blood is different from
venous blood when it's changing, as after a meal. So round off to make
values easier to comprehend and don't sweat the hundredths place. For
example, 4.3 mmol/l converts to 77.4 mg/dl but should probably be quoted
as 75 or 80. Similarly, 150 mg/dl converts to 8.3333... mmol/l but 8.3
is a reasonable quote, and even just 8 would usually convey the meaning.

Actually, a table might be more useful than the raw conversion factor, since
we usually talk in approximations anyway.

    mmol/l     mg/dl     interpretation
    ------     -----     --------------
      2.0        35      extremely low, danger of unconciousness
      3.0        55      low, marginal insulin reaction
      4.0        75      slightly low, first symptoms of lethargy etc.
      5.5       100      mecca
     5 - 6     90-110    normal preprandial in nondiabetics
      8.0       150      normal postprandial in nondiabetics
     10.0       180      maximum postprandial in nondiabetics
     11.0       200
     15.0       270      a little high to very high depending on patient
     16.5       300
     20.0       360      getting up there
     22         400      max mg/dl for some meters and strips
     33         600      high danger of severe electrolyte imbalance

Preprandial  = before meal
Postprandial = after meal

More conversions:

To convert mmol/l of HDL or LDL cholesterol to mg/dl, multiply by 39.
To convert mg/dl of HDL or LDL cholesterol to mmol/l, divide by 39.

To convert mmol/l of triglycerides to mg/dl, multiply by 89.
To convert mg/dl of triglycerides to mmol/l, divide by 89.

To convert umol (micromoles) /l of creatinine to mg/dl, divide by 88.
To convert mg/dl of creatinine to umol/l, multiply by 88.