The objective is to conserve as much of this heat as possible. The same principle applies to your spa.
Many modern spas have well-insulated shells.
If your cover is in good condition, make sure the tie straps are snug and latched when the spa is not in use. This will reduce heat leakage. A waterlogged cover will have almost no R-value, greatly increasing energy consumption from heat loss. A quality replacement cover will not only save you huge energy dollars, it will also last longer, be more resistant to water absorption, and perform its job better. Cover qualities and prices vary greatly, so shop carefully and get all the facts first.