Because Sapa is at a mountainous area, most of the time it is rather foggy and/or rainy. According to most people, the best months to visit are in May or September because the weather is good and the terraces are filled with rice. December to February can be very cold and foggy due to winter time (however I went in December 2006 and was very lucky to have nice and clear weather). In fact, it even snows in Sapa every few years and I have seen photos of Sapa in the snow from a photography book while at the Hanoi airport. As such, bring enough clothings, rain attire as standby and proper walking shoes for trekking. Most of the terrain is in open space so beware of the sun and radiation.
When catching a train from Laocai back to Hanoi, you need to set off from Sapa as early as possible. This is because the bus or van takes one 1 hour to get from Sapa to Laocai (and sometimes this takes longer because the bus or van will wonder around the streets to get more passengers, thus wasting precious time). Once reach Laocai, most people will have their dinner there before catching the night train. Be very careful here because the restaurants here are very slow and they can be overwhelmed by many people trying to eat dinner at the same time. Remember to tell the waitress repeatedly that you are catching a train, and they will then try to serve you dinner faster. If not, they will take their own sweet time and you may land up without dinner before catching the night train.
Sapa folks dressed daily in their colorful traditonal clothes are great for photos. However, many are tired of being captured on camera from daily hordes of "paparazi" tourists. Many actually turned away, covered their faces with their hands, even ran away or signaled me a big "No".
Some will let you take their photos and then expect you to buy something from them. So if they are selling something, do buy something inexpensive to make it a win-win situation.
Others will be happy to let you take a photo if you ask. Just smile, point to your camera and use your hand signal to show your intent to ask for permission before shooting.
Note: Just imagine, if you are out gardening or going shopping and a group of out-town tourists start taking your pictures because you look different from them.
Some longer treks are a few hours and it is in the open along rice terraces with no shade. You can see why the locals wear traditional headgears and long sleeves to protect against the sun.
So be wary of sun burn and heat stroke in the summer.
Wear a cap/hat, carry an umbrella and use sun-tan lotion. Check with the guides, there are options of taking a break at one of the village home, or arrange for a motorcycle or jeep to continue your journey forward or back if you decided you have enough sun.
The roads up in the mountains are narrow, bumpy and winding. Some drivers however drive like they are in a F1 race. This can be really dangerous especially when there is oncoming traffic or livestock on the road. Either close your eyes and pray for the best, or politely ask your driver to slow down.
There are many trains travel from sapa to hanoi. Some are meant for locals, some nicer and more expensive ones are specially decorated for tourists. Beware of the type os ticket your are buying (tourist/local train ticket), the price varies a lot.
It is best to buy a soft bed berth night trian to sapa(4 in a room is nice). We used Ratraco or Et-pumpkin both clean and nice. Victoria express is best if you dont mind the cost.
May be safer to buy Sapa train ticket from a reliable travel agent or reliable hotel (reputable one) to ensure you get what you pay for.
If you prefer easy way out, you may just join a reliable travel agent from Hanoi to Sapa.