If one doesn't have much opportunity to go shoot over the winter, how can they maintain their muscles? Which exercises are suitable? What equipment might be handy?
Sit-ups won't work, archery is all about the shoulders, not abs. Also it's a bad idea to dry aim a bow, it is generally frowned upon.
There are exercise tools you can buy such as a bow training exerciser:
Another option is a training band, such as this:
Failing that you can get yourself a press exerciser, I have one and it is very useful:
Naturally another option is to find a club with a hall. The ones that I belonged to had a hall for bad weather and the field for when it cleared up.