There are various options for extending the season with high tunnels.  The amount of time and care you want to put into your high tunnel will be the greatest factor determing the best form of winter or off-season management.  The easiest is to simpley take the cover off your high tunnel. This helps control pests and prevent salts from building up in the soil, but limits your capacity to warm your soils up during the off-season.  If you travel in the winter, work so hard in the summer that you don't want winter work shoveling snow and caretaking plants, or if you have a high tunnel that can't hold a snowload, taking the top off is your best choice.

But if you keep your top on and want to continue growing as early and as long as possible, then it gets more complicated.  There are various forms of heating and heat retension that can be employed to give your plants a boost as well as varieties of plants that are frost tolerant and winter hardy. Here are some various sources of information found online:

• In the publication "Controlling the High Tunnel Environment" found on the USDA/NRCS Alaska publications webpage, there are details and considerations about heating and heat retention starting on page 6.
• One of the best known four season farmers is Eliot Coleman in Maine.  There are various books (Winter Harvest Handbook, Four Season Harvest) by Eliot as well as many videos of his lectures. One video specifically interviewing him about his winter management techniques can be seen on this episode of Growing a Greener World (skip the first 2 minutes to get past the comercials).