A waterproof shell jacket is a must when you need to get that workout in during miserable conditions because they feature a semi-permeable membrane on the inside that lets moist air our, and keeps heat in. Look for something in the 20,000+mm rating to dependably keep you dry. Remember though, membrane jackets only work if you're putting off heat - so if you're cold, you might need a simple $3 plastic poncho to keep water out.
Tights are a must to protect muscles, preferably a pair of regular lycra and also a softshell pair for the really cold days.
The best baselayers are increasingly wool-poly hybrids, but if you can't get your hands on wool, a thin polyester shirt with high wicking qualities will do the trick.
Make weekend trips to sunny desert areas for Vitamin D boosts.
Get out even on overcast days, as there still is sunlight percolating through the clouds.
If you sport is more active in the summer, create a base plan to peak for summer performances.
Stay healthy so you can still get outside!
Get your rest, make sure any binge TV watching is limited by workouts and getting to bed in time for 8 hours of sleep.
Eat your veggies! Your body might naturally crave more savory foods in the winter, but keep your plate green and colorful before you get to comfort foods. Also, eating veggies before carbs helps slow down the body from converting carbs to fat.
Take a vitamin B12 supplement to keep energy levels up. I usually take a high potency pill every couple days to fight the urge to hibernate all winter.
When your schedule gets hectic, keep healthy meal alternatives around like PROBAR MEAL Bars to get real nutrition and Omega Fatty Acids.
Trail maps at your local outdoor recreation store should give you access to a previously unknown network of trails and natural resources in your surrounding area. If you live by mountains or forests (which many people surprisingly overlook on their way to work), you'll be pleasantly surprised to find some amazing trails hidden from plain sight.
If you do usually hike, bike or run in the summer on trails, buy a pair of snowshoes or ski mountaineering/AT touring skis to cover more ground. Ask around at your local shop for tips about what gear works best in typical local conditions.
Look up events like Hot Chocolate runs, trail races, obstacle course races, and outdoor group workouts.
The internet has made it easier than ever to find "fight club" like groups that don't advertise extensively, but do offer group workouts or events that make it a little easier to get up early when there's a group of friends waiting to see your smiling face.