An acquaintance of mine asked me this question and I will try to simply it.
1. Wear a T shirt
2. Wear long pants as opposed to shorts!
That being said, in cold weather one may need to warm the body so better to rely on appropriate foods (dietary) to increase metabolism.
a. Physical activity is good for raising metabolism
If one is always "cold" then seeing the family physician is in order.
A complementary method consists of the following;
a. Standing Baduanjin
b. Sitting Baduanjin
At the end of one or the other, keep hands at qihai for 15 minutes then on completion. dry wash face, neck, rub hands together and follow a similar pattern of shougong routines espoused by one's teacher(s).
Motive and behavioural attitudes also help to calm the mind thereby increasing the body's warmth. Less ego and corresponding constructs, mental and physical blockages alleviate the attachment(s) so increased circulation.