Flying in winter keeps you more current for when the spring thermals arrive.

It creates some challenges too: shorter days, lower cloud base etc, making for fewer flying days and the temptation to fly in marginal (or beyond) conditions.

The colder air at ground level may be more dense and reluctant to mix, so whilst your anemometer may read an acceptable speed, it could actually be much stronger once you take off and gain a few feet.  

Also consider the possibility of wind sheer, where slow \ stationary cold air sits in the valleys and faster moving air rips over the tops of the hills. Windy gives wind speed forecasts at altitudes as well as at the surface. Metcheck paragliding forecast does too. There are no doubt others.

In the denser air, a small change in direction sometimes results in a disproportionate increase in turbulence, and the turbulence from upwind features propagates further downwind.

Needless to say, clothing is important. For those of you in your first winter flying season, consider the following to delay \ avoid turning into a flying Popsicle:

A pair of really warm gloves
Sturdy water resistant boots, thick socks
Thermal underwear
Several layers of clothing (maybe check your all up weight with it all on and maybe adjust your harness shoulder straps before takeoff)
A balaclava under your helmet (if it fits)
Take a flask - but leave it in the car - they do not make good back protectors

The advice to fly only with others holds even truer for winter days where time would be even more critical in the event of an accident.

And maybe think about the road conditions before you set off.

If the weather precludes flying, or you don't fancy freezing your bits off, well, it's a good time to get your glider sent off for inspection and service.

Fly Safe
Safety Officer