I've read a few places that you should not do resistance training on the same muscles two days in a row. When doing resistance training you are actually breaking down muscle. Building muscle and burning fat occurs during the rest period - 24 to 36 hours.
That's my excuse for a rest anyway
Seen a great formula online, can't find the site anymore but it was something like this:
Resistance Training + Cardio + calorie control = success!