I've never had too much of a problem with condensation inside down from me (i.e. having the down wet out) due to frost in it
until I get down to very cold temperatures. (well, something like -10 to -15C and below. If it's above that I tend not to worry about it too much. If you do have to worry about it, a vapor barrier liner will work (as OM mentioned) if you can stand to sleep in them. If you can't you can use a synthetic overbag - the goal here is to keep the line of freezing outside the down bag, so the frost collects in the synthetic overbag. Works for me.