I agree with Chris, the practice card system will help with individual behaviors. I also work with small group behaviors by having students working towards having the privilege of a "special center". For example, groups that can maintain whisper voices OR can manage their own people (for example, a student forgets, but the rest of the group says (in a whisper voice: "Please whisper"), then they get to have the privilege of going to the special center that day. Right now we have our "Environmental Print" (aka: Kitchen Play center) as our special center. And yes, they do have to whisper talk in that center:) . If you can't have dramatic play centers (as some school districts are no longer allowing this), find something fun/ educational and hype it up so that they want to go there. Also, change it up frequently to keep it exciting! I work the card practice system within that system. Students who cannot help their friends maintain whisper voices AND the student who did not whisper get a picture practice card. Then at recess, they practice whisper voices. They either say "I can whisper in a whisper voice" over and over again for 2 minutes, or they say "I can help my friends whisper" over and over for 2 minutes. Remember, this is not a punishment--this is just "extra practice time." Students have the choice to practice following the rules correctly during class time, or during their recess if they need extra practice time. IF the students lost the special center privilege, they get the word "no" written next to their group. They have to work to get that "no" erased at their next center time, if they want to get that privilege back. Also--just a note--the way my rotations work, the special center is only once every two days so if they lose it, they may lose it for more than one day. But if you rotate all the way through daily, then they should only lose it one day.
Hope this helps, but as you recognized---consistency is going to be the key. It's never to late to be consistent:)!