The best thing you can do is PREPARE. This has been said earlier, but it's so important for a beginning instructor. Write out your examples, mock up your speeches, and anticipate difficulties the night before your classes. After you teach for a while, save some time by mentally reviewing, fully solving only the more subtle ones. Remember that a problem's difficulty lies not only in its execution but its presentation! The effective communication of your work is key.
Speaking of communication...
Boardwork. Think of how your best professors use the board, including their use of whitespace and delineation. It's an art. Like any artform, know your materials: different classrooms have differing setups. Bring fresh markers/chalk with you - using a weak marker can quickly make your lectures feel lukewarm. Make your work vivid: use colors.
Using a board well requires a bit of precognition... You need to predict how a certain concept or example will evolve to effectively use your space. Some information you will want to stay visible through certain examples; other information you will want to make ephemeral, like when you answer a digressing question. You can verbally alert your students to the important or unimportant material. Sometimes you might want them to not copy down notes but simply be attentive.
Your boardwork will be cleaner if you prep, and will hone itself if you reflect. Observing how others' use a board is immeasurably helpful as well; it's like watching an artist paint... poorly or masterfully.
Verbal communication. For the love of teaching, avoid relying on pronouns. Don't solve it - evaluate the line integral from r equals a to b. Don't substitute that into that - substitute the geometric definition of the dot product into the right hand side of the Cauchy-Schwarz inequality. The more accurate you are, the finer you will deliver your point, especially when a student is head down, copying notes. Just think of all those times you wanted to scream your head off when some teacher of yours referred to everything as this: This is vague.
Be sure to explain any technical language, and train your students to speak technically: correct and clarify their questions. It will aid you both.
Repeat yourself. Students are busy assimilating your words with visual cues, scrambling to copy information coherently... They will miss your words and intention, so repeat yourself, pause, and repeat yourself again. Pausing permits processing.
Respect your time. I suggest timing yourself whenever you create course materials and especially when you grade assignments. A lot of your work will become streamlined with experience, but the earlier you focus on your time management, the better. For example, the amount of input you give to your students during grading can consume an enormous chunk of life. Addressing this can amount to a level of restraint. Similarly, office hours should be the students' time, but when office hours end, that means their time ought to end. Give them resources they can access outside class. Remember that they might not yet be strong self-learners, so nudge them in that direction.
Depending on the class, it might be wise to restrict homework questions to office hours and your email strictly to administrative issues. Don't feel uncomfortable setting such rules and boundaries.
And for everyone's sake...
End on time. Otherwise known as respect everyone else's time. Not as easy as it sounds :)