1. Pay Attention. PUT YOUR FRAKING PHONE DOWN! This is my biggest pet peeve at the table. Unless you're using a dice roller, live Tweeting the session, or looking something up from the SRD, show your DM and fellow players some respect and put the phone away. Also, know the initiative order and have your turn planned out before you come up in initiative order. No one wants to sit there for 20 minutes while you decide how you're going move and attack or decide which spell to cast, especially if you've spent the entire round on Facebook.
2. Be Prepared. Have notebook paper, pens, pencils, dice, your miniature, and updated character sheets with you and ready to go.
4. Roll Dice Together. To speed up your turn, roll your hit and damage dice at the same time. Sure, you'll miss sometimes and won't need the damage die, but on the times you hit, it makes the time spent slogging through game mechanics that much faster.
5. Learn the Rules. Now I'm not saying that you have to be a savant and know every single nuance to every rule, but it's a good idea to at least have a basic understanding of the combat and skill systems. Unnecessary rules questions slow down game play. Having a basic understanding of the rules will make the game move more smoothly and reserve the rules questions for more difficult situations. Being a new DM and new to 5e myself, I'm still learning the rules even as we play through our first adventure, but I AM making it a point to learn them.
6. Cut the Chatter, Red 2! Everyone likes to have a good time. Let's face it, that's the main reason you're playing D&D in the first place. That being said, don't spend 15 minutes chatting about what cousin Beavis said to his wife at last Sunday's ice cream social. OK, bad example, but you get the idea. The point is, try to keep conversation on topic. If you have a loose and irreverently silly group, that's fine, but try to keep the irreverent silliness confined to the adventure at hand.
7. Take Notes. There shouldn't have to be 20 minutes at the beginning of the session dedicated to figuring out how many Action Points, Healing Surges, and Hit Points you had after the last session. Nor should you have to spend time figuring out what happened last session. If it seems important, write it down!
8. Don't Meta-game. Meta-gaming is using out-of-character knowledge to gain an advantage with in-character actions. Remember that role play is about what your CHARACTER would do, not what inside information you have as a player. For example, using knowledge from the Monster Manual such as a monsters vulnerability to radiant damage to your advantage even though your character has never encountered the monster before is a classic example of meta-gaming. Always address each situation from the "What would my character do?" and "What would my character know about this?" angle. Knowing your character (See #3) will help you answer those questions. Meta-gaming kills role play by killing the suspension of belief that is required to get into the story at hand.
9. Ask Relevant Questions. When the DM tells you, "A mysterious female Elf steps from the shadows wearing dark chain mail and says 'Halt! State your business!'", "Is she hot?" is probably NOT a relevant question. "Do I recognize her?", "Is there any insignia on her armor?", "Do I see anyone else in the area?" are most definitely relevant. Now, if the same NPC approaches you at a tavern and in an alluring voice says "What brings you to town, handsome?" then yeah, "Is she hot?" might be relevant. Think about the situation and ask questions designed to give your character useful information.
10. Role Play! That IS two thirds of RPG, you know. Get into your character and start imagining them in the situations the DM lays out for you. You're the star in your own private play, seize the role and make it your own. And never ever forget, have fun!