The secret is to use loose layers of clothing, leaving lots of room for air pockets to trap heat where it is wanted. A good winter jacket, preferably down-filled, will keep you warm above, but layers of undershirts, shirts, and sweaters, covered by a winter coat will do. Big friends with large ski jackets will help here, but ski wear is not suitable by itself to handle the low level of activity when observing in the cold.
Feet deserve special attention, but the technique is similar to other parts of the body.
Try to keep it loose if possible to trap warm air and allow your feet to move and warm up. Don't neglect the underside of your feet - pick boots with a good thick insole to block cold coming up through the bottom. Gloves should be thin to handle the small parts of the telescope, but your hands will probably be cold nevertheless. Thin gloves under a larger pair of mitts work best. Watch out for cold metal surfaces on eyepieces and mountings.