1) It is important to keep your feeders up and well maintained and your gardens blooming as long as possible. You will not prevent a hummingbird from migrating, but your efforts might help a hummingbird survive.
2) If you have a hummingbird show up off season in your yard (either a very late summer resident or a hummingbird not usual to your location), contact your local bird watching group (usually an Audubon society), both to make them aware of the bird and for professional identification and documentation.
3) Collect as much information as you can on any late season hummingbird that shows up in your yard: get photos if possible; describe it in writing as well as you can; identify it if possible; note the date when the bird first appeared and when it was last seen; record any daily activity patterns of the bird, such as when it visits the garden/feeder and how often.
4) Notify hummingbird researchers to report wintering hummingbird(s) and to find out if there is a person who is permitted to band hummingbirds in your area.