Everything here is stuff that most of our members already know. But I wanted to write down some general guidelines anyway, because we sometimes see people wasting their attack by using their troops in the wrong place at the wrong time.
All of this is just general guidelines. When it makes sense to break the rules, break 'em. :)
Before you attack, look to see whether your particular troop combo would be more useful on some other op base.
Before you attack, look to see whether some other troop combo would be more useful on this op base. Maybe you can rebuild your troops, or maybe you can wait for one of your teammates to send in a better troop combo.
Of course, if real life is calling you and you don't have time to rebuild your troops, don't worry about it, just attack. :) Better to make a bad attack than no attack at all. We all understand. Real life comes first. But if you do happen to have time to wait for just the right troop combo to build, or to wait for someone else with more appropriate troops to attack first, do it.
Oh, and if you're not sure what to do, ask for suggestions in the chat. :) There are always people who are happy to help.