If I were to advise one thing for those planning their training for Marathon des Sables, I would say get used to long days out hiking/running with a heavy bag.
Do this and you will be fine. If you can do this in a hot country, you may even be a contender for a good place.
Otherwise this is what my training looked like and how useful I found the strategy to be:-
Running with a heavy bag
I think it's important to get used to this, but don't do all your running with a bag. You'll lose your technique (and your love of running, trust me).
I had virtually no issues with sore back or shoulders. I had no chafing problems either. Some people had horrendous sores on their back.
See also what equipment I took. My bag weighed 9kg. Get down to this or less if you can, particularly of you want a place.
If you are serious about a top 200 place your bag must be in the 6.5 to 7kg range.
50 mile run
I managed one 45 mile race, the Country to Capital. I wanted to fit in two but in the end felt I wouldn't have enough time to recover (I only trained for 6 months prior to the race).
The race helped me but more in a mental way. It was hard! but I knew I could do it.
I would aim to get in two long races if possible. Maybe aim for one fast time, and one with hills and a pack.
Back-to-back long runs
Essential. You need to get used to running long miles day after day after day. I built up from running into work everyday (4.5 miles) to three back to back 25 milers.
I think these were important as they help you get used to the right pace. Go too fast and your muscles will be ruined to do a good job the next day.
Unless you can get out to a hot country and train, this will be a problem. I thought I'd be ok as I love the sun, but 50 degrees is 50 degrees.
I spent two weeks prior to leaving doing sweaty yoga, stretching in the sauna, having hot baths and exercising in my flat with the heating up full whack!
I still suffered from sausage finger syndrome and felt fairly nauseous out there.
If you've got the time (and money) invest in some heat chamber sessions as well as doing all of the above. I would also try and get out to the desert a week beforehand and acclimatise.
This didn't really trouble me too much. Many sections are hard and rocky which is something to bear in mind. Training down in Mertyr Mawr did prepare me for the dunes though and I would recommend this, even if it is just to test your trainers and gaiters don't let in any sand.
Long days outdoors
In hindsight I would have added in more weekends away out hiking outdoors in hilly locations. This is great preparation for the Marathon des Sables. It also gives you a chance to have a break from running, and gives the joints a bit of a rest.
I managed to get in a fair few hill training sessions - some short, fast, others long, slow. I tried to do 1x specific session a week, and at least 1x run that included hills. However on reflection, I don't think I did enough slower, long sessions with a bag.
Now I've completed the Marathon des Sables, I would say hill training is essential, essential, essential. There aren't hills everywhere, but there are a fair few - and they are big. There are also all the dunes, and the slow, steady inclines to factor in.
Some hills you will have to walk up. Others you can shuffle up. I found I did overtake people up hills as my legs felt strong.
Targeted strength training
Your legs have to be strong. Especially your hip abductors - your key stability muscles for walking, and also supporting your bag.
I made sure I included at least 2x week targeted strength work. I would recommend this for all runners though. I've never done this before - I just ran - and I noticed the difference. I would start this as early as possible as it takes a while to get the right technique, and build up the strength.
Strength work also helps you identify potential weak areas, which helps prevent injury too.
Core work/Flexibility and stretching
More of a generic one for all runners again, but I tried to fit in 3x week core work, with 1x week yoga or pilates session. I found my joints became very stiff with the increase in training, so the pilates and yoga really helped this. I was also religious about stretching after a run.
High weekly mileage
I ended up only racking up an average of 40 miles a week (see right for my tally) whereas I thought I'd do more. In hindsight I think this was okay. I've read blogs of people who do less than this and finish. Again I guess if you want a good place you should do more miles.
I struggled a bit with knowing when to taper. In the end I went for 3 weeks before (though I did run a marathon in my taper. Ha!).
I think a lot of people underestimate the effort your body goes through (unless you normally train in this way and this number of miles).
I felt really good at the start. My muscles and joints were relaxed. I had spent the last two weeks doing a lot of yoga and stretching which was also beneficial.
Short, fast runs without a backpack/speed sessions
These ranged between 3-10 miles. They are not necessarily specific training to the Marathon des Sables, and I wouldn't concentrate too much on them, but they do help add miles to your weekly total, and break up the monotony of the super long days out!
If you are going for a place then these will definitely be more important to you. I'completely lost my speed. 12 minute mile pace (as opposed to 8) is my comfort zone at the moment, but I can jog at this pace for a long time!