By chance I just arrived from a spotting trip which included, among others, MXP
When 35R is used for landings a good start is the parking place/playground located on the east side. Photos from sunset until around 1030/1130LT depending on how strick you are with light conditions. From 1130LT to around 1500LT there is no option, the sun will change sides and remember that at 1430LT they switch runways, ie. 35R will now be used for take-offs and 35L for landings. In this case you can go to the Strada Provinciale 14 spotting point, close to a crash gate. From there you can make nice approach photos until sunset.
When the day starts with 35L in use for landings you can go either to Strada Provinciale 14 and choose a place to stay or, like I did, go to Via Gaggio, which is a small gravel road further south. I prefered this location as it is easier to park the car. As 35L is used for landings, you can also opt to make only departure (line-up) photos during the morning on the spot I mentioned for 35R above. Please keep in mind that 35L arrivals will be far away even for digital cameras with crop factor. Again, I found the light good at the Via Gaggio spot good until around 1100LT.
During the afternoon, when they change the runways again, you can go again to Via Gaggio for 35R arrivals, just park the car further down. This place is good as it offers a lot of shade, so it is cooler than staying in the open at SP14. If you would like to do some departure photos, there is a spot close to the Holiday Inn Express Milano Malpensa, near to the Alitalia Flight Operations parking area, where you can do some decent take-off shots. Keep in mind that some lamp posts can be in the way, specially with heavy departures.
During my stay I used the Ascot Lodging Hotel, in Cardano al Campo. It is very simple but nice hotel, good enought for a spotting trip rest, but this is a matter of personnal taste.
During my visit the runway use was: 05 for landings, 35L/R for departures.
During the morning hours the place to be is the IBB Florya park. Actually I did not spot from the park itself but from a walkway along the seaside. No bothering at all and you can start around 0700LT until 1130/1200LT.
From 1200LT until 1430LT more or less, the sun is changing sides and you can go for a lunch or visit the aviation museum close to the airport south perimetre fence. During the afternoons (1500LT until sunset) I used only the FlyInn viewing area. This is in fact a smokers area (the only available for the mall) so if you are a non-smoker (like me) beware. The area itself divided in two.. the right side is a general use area, free to all, with some tables and chairs. The left area is reserved for the customers of a cafe and this area has nicier chairs and also not so many smokers
They have no objections to photographers at all, just ask for a tea or coffee from time to time (you may also try one of the cakes they have, all are very nice).
The "bad" news is that you will see a huge ammount of Turkish Airlines, Atlasjet and Onur Air aircraft, maybe 80% of the daily traffic. The good news is that the other 20% are made of many ex-CIS airlines, Middle Eastern and the occasional odd-ball. I only saw MNG Cargo arrivals during the morning, they have three or four arrivals within 40 minutes, between the never ending flow of THY.
Feel free to ask anything...