Here is a great chart that analyses all the temperature data collected in Vermont over the years and gives us the probability of dates for first and last frosts around the state. South Hero and the lake Champlain Islands have Vermont's longest growing season (measured by number of frost-free days according to N.O.A.A). Take a look and prepare your garden for those first frosty nights. Here is what I do at my house to prepare for those first frosts:
If it is a really early frost, (September in Hinesburg), I will harvest all the ripe fruit on the heat loving plants (squashes, tomatoes, peppers, eggplant, etc) and if the plants still look healthy and have lots of unripe fruit, I will cover the plants with row cover. You can use sheets, blankets, plastic sheeting, etc. If the frost is coming at a later point, I often won't bother with the plant protection - just harvest all the fruit (green tomatoes can ripen indoors) and call it a day!
I don't bother to protect lettuce unless the first frost is very cold and very early. Lettuce can handle a few light frosts, so it is usually not a problem early on. Later in the season, I set up some simple wire hoops and keep the lettuce under row cover for the remainder of the season. This allows fresh lettuce to be harvested for salads well into November. It is good idea to seed or plant fresh lettuce in late summer and early fall so that the protected plantings of fall are fresh and tender. It is not really worth it, from a culinary perspective, to keep old or bitter lettuce alive. Plus it won't do as well if it is past its prime and won't fend off the cold like a younger planting can.
Hardy greens like kale, mustard greens, collards, etc do not need row cover and can live, unprotected, into December. You can always put some sort of protection over them in November to increase the harvest period past December, but it can be difficult to do with the taller plants. The wind dessicates them and makes them unappealing, and without a larger structure like a cold frame or mini-greenhosue, it can be difficult to give them adequate shelter.
All these attempts at fall crop protection will leave you grateful for the sweet rewards of fresh salads, tomatoes coming out of the cellar ripened and tasty, and nutritious leafy greens sweetened by the kiss of cold.