Haggling can be very stressful and you should take a light hearted approach to it and remember that nobody can force you to buy anything that you do not want.

Arrival 

There is a Police form to fill in at the airport before you get past Passport Control so make sure you pack a pen, you then get to fill in a similar form at your residence and again on leaving, its also in French and Arabic just to make life a little more interesting. Don't forget to get your local currency at the airport.(the exchange rate is slightly better than the cash machines) There are a couple of counters half way up the arrival/departure lounge on the right, Ask for a few smaller denomination notes as most taxi drivers never seem to have any change

Shuttle bus from the airport

 

There's a bus that will take you to near Djemaa El Fna.

Taxis From the airport

At the Airport you will find an official pricing board.

However, none of the taxi drivers take any notice of this board.

The official price for a taxi from the airport to The Djemaa El Fna and most of locations around the city is 70 Dirham during the day and 150 Dirham at night. However, the taxi drivers will ask you for much more. If you refuse to agree to their price, they may simply refuse to take you. However, you should approach some more taxi drivers and you may find one who is willing to break the cartel and take you for the price that you want.

Before you get in any taxi agree the price first as most Petite taxi drivers in Marrakech will not use the Meter. 20 Dirham may take you to most places within the Medina. 

Djemaa El Fna:

When visiting the Djemaa El fna, enjoy coffee or lunch on a roof top terrace and watch the evening food stall holders set up shop (they appear with their barrows from 4pm) then stroll around the Souks until is time for dinner.

The food markets are open until around 11pm each night and there is plenty of entertainment around to keep you interested. The stall holders will try to get you to look at their food but if you are still not sure of what you want to eat and where then simply say you have just eaten and they will generally just tell you their number so you can go another night and let you move on.

The orange juice sellers are also there until late and for 3Dirhams you get a tall glass of freshly squeezed juice made right before you.

The Souk

Expect a lot of attention and greetings from everyone. "Asda price" is a common thing the shop keepers shout out to get your attention when they think you are British.

Decide how much you are willing to pay for what you want to buy.

Do not be scared to offer a reduced price, but do ensure that your offer is fair.

Do not waste time haggling over 10 or 20 dirham.

Do not be scared to walk away, out of the shop.

It is likely that the shop owner will call you back and negotiate properly with you then, once he sees he could lose the sale.

Do enjoy the haggling and have a laugh with the stall owners, a smile and friendly attitude will gain you a friendly hello next time you pass that way.

One user states that the shop owners first offer for a tea pot and 6 glasses and tray was 400 Dirham.

The deal ended in the user getting the product for 110.

Be aware that the shop owners first opening offer may be over inflated, so that they can negotiate down to a decent price. 

Make sure that you have a good map, the not very accurate tourist ones that the Riads issue are spotted easily by the locals who may then 'help' you to find your location for a small fee and via their uncles shop.

Women traveling alone or another female friend

Hire a guide to any attraction from your hotel if possible. It's best to dress with sleeves and long pants. Asian travelers- expect commentary and other sexual words directed towards you. Just ignore and keep walking. 

Do something out of Marrakech 

From Marrakech you can go for a day trip to, Ouirgane, Ourika Valley or Ouzoud Waterfalls. Essaouira can be a day trip but is better as an overnight visit. If you have 4 days to spare you could go for a Sahara Desert Tour including camel ride etc. check  Marrakech forums or activities to find the best tour company/ guide for this trips .