It is easier to start out tough and lighten up later than to do the opposite. Grades can have such a big effect on students' fortunes. In our experience, there is little question about who deserves the As and the Fs. It's everything in between that is difficult. Grade and return assignments quickly.
Quick feedback is more effective than slow.
It shows the students that you care about them. If you are overloaded, let your students know that the return will be delayed. Students will rise to your expectations. The first time you give a zero for missing a quiz or turning in an assignment late, the word will get around and you will not have many more of those problems. Later, if you feel that it will not create problems, you can mellow out a little and quietly give them some slack.