Like it or not, it will often be the case that you get attacked while you are away and your shield is down. Sometimes you will get plowed, and that sucks – but getting attacked also presents a rare opportunity!
When reviewing the list of punks who defaced your village while you were away, you have the choice to “Revenge” – to attack them back.
This is a huge strategic advantage, because you can look at their village before attacking, and plan your attack – even train different units for a special assault. I recommend checking their storages (which you can do visually, or better yet by clicking on them and checking their levels), and also the location of their Town Hall.
Since you may have a big list to go through, check farmers first (folks that attacked you with just a few units then gave up) since they are likely to have big resource stockpiles. If an opponent is shielded or online, just check back later.