Holding - it does a body good
Many people hold treats out for their dog with the tips of their fingers. Again, that's not wrong but if you have a slightly aroused or anxious dog, you are increasing your chances of inadvertently getting your fingers bit. Your cuticles will thank you for holding the treat differently :). I recommend holding the treat in your palm and using your thumb to keep it there. This also allows you to form a small cup with your hand allowing your dog's nose to push inside for the treat, even though they can't actually reach the treat until you let it go with your thumb. Or if you are using tasty human food as a reinforcer, you can do what I sometimes do - spit it at the dog :). Of course please work on your spitting skills beforehand. This helps reinforce your dog to look at your face instead of focusing on your hands.
The delivering that smiles back
There are a few different ways to deliver reinforcement and which one you choose will depend on your training situation and your dog: 1. delivering it right into your dog's mouth, 2. delivering it certain spots on the floor (excellent when doing shaping), and 3. or having your dog catch it (think toy reward here especially). When I am working on a stationary position (like sit, down, stand or stays) I like to deliver the treat directly to my dog's mouth. This helps reinforce what I am rewarding them for and helps them not move out of that position :). Another time when I would deliver the treat directly into the mouth is if I was working on desensitizing or counter-conditioning because I want the dog immediately rewarded and rewarded in a calm manner. As mentioned above, when I'm working on shaping a behavior I will strategically reward my dog in certain places on the floor so that they reset themselves to continue building the behavior. And lastly, having your dog catch a treat is great for building motivation or drive (as it somewhat simulates activities relating to prey) and increase their excitement level in participation.
Any time, any place, positioning
In many cases the positioning of the reward will help your dog clue into the final goal much faster, or as mentioned above, keep them in a certain position. For example, if you are teaching the front position (where your dog comes and sits in front of you) and you always reward them with your right hand, your dog will start sitting crooked towards your right hand. It won't even matter that you are clicking or marking the correct moment and have great timing. The presentation of the treat will shift your dog's position and they, of course, are just making it easier for you to find their mouth :).
Presentation of the reward (as much as possible) should always be done in a way that is reinforcing or stabilizing the behaviors you are working on. You should always make it a habit to reward your dog with either hand, interchangeably, so that your dog knows that rewards come from both side and it will help cut down on anticipation position shifting.
One last thought
Another thing to remember is that the delivery of the reinforcement should be a distinct, separate action from your click or marker word. If you click the clicker (or say the marker word) at the same time you are moving your hand to deliver a treat, in most cases your dog's focal point will be on the movement of the treat hand. The sound of the click (and information you were conveying with click) will be diluted or blocked altogether.