Most of my emails and tweets in regards to questions tend to be MAC based which is cool
with me as I am one of the biggest MAC fan girls around...sorry I'm not sorry!
I posted an original guide over a year ago but felt it needed updated a little.
I am no expert but due to the hours I have spent in numerous Mac Stores I have picked up
some information along the way which may be helpful to some.
Like I said i'm no expert so feel free to add your own tips/info in the comment section,
the same applies if you have any  burning questions - hopefully someone will be able to help.

MAC Foundation Shades Explained 

This is the one area that seems to confuse people and I was no different in the beginning, 
with so many conflicting views on what NC and NW actually mean 
and who should use what it all becomes very much a grey area.

I went to the source (I emailed an MAC Artist) in order to provide the best information possible 
and I can confirm the following:

NC - Is the cool toned foundation shades and have a yellow undertone
NW- Is the warm toned foundation shades and lean slightly pink.
N - Is the neutral toned foundations, some of us will be slap bang in the middle
with no warm or cool undertones.

In theory you should use a cool foundation if you have warm skin and warm foundation
 if you have cool skin but I find the best way to determine what shade would best suit you is to go in-store
and swatch a good few shades, then ask for samples of the three that YOU find best suits you.
I say YOU as sometimes the assistants in store can be a tad bullying
and steer you towards a shade that is a little too dark etc.
Test out the three shades and then decide which best suits your tone, remember the perfect shade will completely disappear into the skin - the jaw bone is the best area to swatch foundation.

Face and Body foundation is a little different it comes in three catagories:

N- Neutral
C- Cool (Equivalent of NC)
W- Warm (Equivalent of NW)
Neutral is for those who have neither warm nor cool skin, it is sometimes deemed a suit all shade.

It can be a difficult task to determine if you have warm or cool undertones 
but you can try the following methods:

Vein Test - Look at the veins on your wrist see what colour is most prominent:
Blue Veins = Cool Toned
Green Veins = Warm Toned

Silver or Gold - Some say that if you suit gold jewellery best then you are warm toned and if silver suits you best you are cool toned.(I'm not a fan of this method)

Back to MAC

You may be aware that if you return six empty mac products to a Mac store they will exchange it for a lipstick of your choice (in the US/Canada i'm sure you have the choice of either a lipstick or eyeshadow). Mac are rather specific on what you can return but if you check the box the product comes in, it will have the back to mac logo which means you can return it for the back to mac programme.
In-case you have lost the cardboard packaging as a general rule you can back to MAC most full size products this includes limited edition products, skin care products and believe it or not the full size face wipes as long as the empty pack has the plastic enclosure.

You can not return any mini products such as the small pigments you get in the holiday kits nor can you return any Viva Glam products or the sized to go products. If in doubt call up Mac and ask.
Most MAC's will not accept depotted blushes and eyeshadows, MAC now check that you return the packaging in full, meaning they must have an empty silver pan as well as the black casing.

MAC and eBay

To be blunt - best avoided! 
The sheer quantity of fake/replica Mac products on eBay is beyond ridiculous. 
I'm not saying every product on there is fake but the vast majority is, since the product is counterfeit do you really think the retailer cares if the product is dangerous and could potentially harm you? NOPE!

There is ways of identifying the real from the counterfeit but the fakes are getting better with each batch they produce. There are plenty of guides on eBay about how to identify fakes such as this one which provides comparison photos and is a must read for anyone still contemplating purchasing anything Mac related from eBay. If you really must purchase a Mac item from eBay I highly suggest Google searching for the item and comparing the shades, packaging etc.

MAC Dates/Batches

If you look at the back of any mac item you will see a letter and two numbers, the letter stands for the batch number, A= batch one, B= batch two and so on.
The numbers determine the month and year, the first number determines the month (1 being January, 2 being February and so on) and the latter the year so the lipstick above has the code A31 so it would be Batch one, Month March and Year 2011.

MAC launch dates are fairly easy to predict...well roughly anyway.
They tend to be the first week of every month and normally a Tuesday but that can change as it isn't
official nor set in stone.
The January collections of each year have went live either on boxing day or the day after in the UK.

This has turned into a rather lengthy tome but I will have the second installment up tomorrow
explaining various formulas and finishes but if you have anything else you wish to see included
do let me know and I will try my best!