1) Get enough sleep: it's already hard to wake up early on a cold dark morning, or step on your mat after the sun has gone down. A well-rested body is absolutely key!
2) Brighten up your space: turn the lights on, drape furniture in bright cloth, wear bright colours, play music - add some cheer to your space. This really helped keep me going for my morning practice - I don't usually practice to music but in the winter I found it absolutely life-saving!
3) Add some heat-inducing pranayama to your practice, such as: Ujayi breath, Udiyana kriya / nauli, bhastrika & kaphalabati. (If anyone wants more info on those, just ask!)
4) Warm up properly! In winter (especially with a morning practice) I found I really needed to warm up the body more. I added sun salutations to my warm-up, and kept up a brisk pace through my practice. If you have a flow practice with a varied routine, winter is a great time to add dynamic standing poses, balances and twists that generate heat from the core, like Utkatasana (chair pose), parvritta trikonasana (reverse triangle), parvritta parsvakonasana (reverse extended angle pose), Warrior III, Garudasana (Eagle Pose), Plank Pose & Side Plank Pose. Be more cautious about going into deep forward bends, hip openers or backbends, since your muscles may not be as warmed up as they are in the summer months - take lots of time to warm up to these. Take vinyasas between sitting poses to keep the body heated, and keep some warm socks and a sweater / blanket ready for your meditation and savasana.
5) Modify your diet: Eat warm, cooked foods in winter to balance out the cold. If you are a student of yoga, chances are you have come across Ayurveda, the yogic lifestyle science, (if you need a refresher, have a look at this post), which allows you to look at the elements of your individual constitution, and identify imbalances. Cold, dry winters are most likely to aggravate Vata, and cold, wet winters are most likely to aggravate Kapha, which may cause imbalances. If you feel that winter throws you particularly out of balance, it might be worth delving deeper into your ayurvedic type and adopting some of the lifestyle recommendations specific to you. Ayurveda also offers some suggestions for what type of physical practice might suit you best or balance your imbalances.
6) Practice at different times: Some people (like me!) really benefit from having a regular practice time and sticking to it, but others may find that varying your practice time can help you maintain a practice during the winter months. If you are able to, try practicing at different times of day and see what suits you best.
7) If you are teaching in winter, you can also include many of these tips in your teaching routine. Winter classes can be a great opportunity to bring your students into challenging standing sequences, and work on core strength. Make sure your students are properly warmed up by adding heating poses to your warm up sequence, like cat / cow balance (opposite arm and leg raised), one-legged cat/cow (on inhale, extend one leg out behind you, on exhale hug the knee to the chest and engage the core), plank pose variations (one legged plank or side plank), utkatasana with twists. Warn your students to be especially cautious with hamstrings, hips and lower back, and to make sure they feel properly warmed up before trying deep poses in these areas.
And, just for fun, check out this page for some yogis who thought they'd take their winter practice a bit farther and practice yoga in the snow!! Also, check out more amazing snow crystal photos like the one above at Snow Crystals.com!