keeping the head warm is essential, mummy bags are the best but a hat will do though mine always falls off at night.
don't go to sleep drunk it always makes you feel warmer then you really are
the most important are these: drinks lots of water, get some kind of regular nutrition even if it's only the occasional multi-vitamin or smoothie and no matter where you end up sleeping make sure you get plenty of it.
oh and some sort of regular exercise, again, it doesn't need to be much, just a few sit ups and pushups or a few minutes of jogging. everyday if you can.
then if you want to go above and beyond and never get sick ever again.
start meditating (this does not have to be structured, but rather understood, check out j krishnamurti's talks on meditation)
if you smoke tobacco, quit, if you drink heavily, cut back, and when you drink dont drink cheap.
none of this is actually difficult. changing habits and developing new ones, well, there does appear to be a learning curve.