A couple of options for insulating windows without obstructing the view are to add another layer of glazing by replacing existing windows with double- or triple-glazed ones, install storm windows over the existing windows or place sheets of plastic on the existing windows.
The R-value resulting from doubling the glass is actually higher than the sum of the two panes of glass because the air trapped between them acts as an insulator too. However, the air space should be no wider than three quarters of an inch to keep the air still. The second sheet of glass should either have an air tight seal with the first or have condensation vents to let moisture escape.
During the winter it is normal to see some condensation form on the inside of the storm window. This means they are insulating properly. If too much water is collecting on the sill instead of draining out through the weep holes, it may mean the humidity levels in the home are too high. If moisture is condensing between the panes on the primary window, it is a sign that your storm windows are not insulating effectively. There may be air leakage around the prime window or around the storm window.
Replacing old single-pane windows with new double- or triple-glazed ones is expensive, but much less tedious than putting storm windows up each winter. The windows are also more efficient and less likely to have condensation problems. A compromise is installing storm windows permanently on existing windows. They are less expensive than replacement windows and can be sealed tightly in place.
Putting plastic on the windows is an inexpensive but temporary solution. Heavy duty plastic for attaching to the exterior, or more attractive colored and clear interior films, are available in most home improvement stores.