In the past, while I have been very careful, I have used the backfeed method to provide whole home backup power when needed. However I was never comfortable doing that.

When I recently replaced my 1981 main panel I incorporated something I also saw on "This Old House" or "Ask this Old House".

It is two interlocking breakers that prevent the generator circuit from being closed when the main breaker is closed. Said another way you need to open the main breaker before you can close the generator breaker.

However to use this solution you have to be comfortable with managing the loads unless you have a generator that will actually run your entire house.

This is probably the least expensive solution if your existing panel will accommodate the hardware.