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.
Push-ups, sit-ups, dry aiming you bow (do not do this with compound bow, you can derail your strings), one of those stretchy surgical tubing pull exercise gizmos.
Yoga is good exercise for all muscle groups and keeps you limber and supple, not bunched up.