A bit of an exageration to say intra-european flights are always empty. Its a matter of the day of the week, time of day, special events etc. However, depending on the city you're flying into, and I think you want to get to Rome, there's a high frequency of flights per day, spaced at max two hour intervals, so chances of getting on one are much higher. That said, ex Frankfurt to Rome, or Milan, best bets are first or last flight out during mid week, Tue to Thur. Mid afternoon flights also usually leave with room. If you're thinking about smaller cities such as Florence or Verona, as these are serviced by small regional jets, summer travel can be tight. Plus the added disadvantage of just two or three flights per day. Your main challenge in Aug would be getting back and forth across the pond.