I ride single track trails every week for a couple of hours and wonder how often I should tune the bike up. As far as this website goes, it's not a very good bike but it's been great for me. It is a Iron Horse Maverick 4.2, full suspension and disk breaks.
The one thing that winter time is great for is the preventitive maintenance that will hopefully keep you from any holdups or layups when riding is prime in the summer. It's a great time for shock rebuilds, or even just new seals. Also, have a shop look at the brakes and see if they need a bleed or pads. Along with a regular tuneup, it may be worth repacking the hubs and headset and maybe even replacing the bottom bracket. As for me, I always replace my cables in the winter time just to make sure everything is good to go.