Sitting in 2 steps.

I have mentioned using a treat for reward.  A compliment a pat on the head can work as well.  The objective is simply achieved without hitting, rubbing the nose in it, kicking or any other form of cruelty.towards an animal.
You have to get in sync with your canine buddy if you are to master the dogs behavior.  Besides dogs having differences based upon what they were bred to do, each dog within a a specific breed, has its own personality.
Chevy our youngest, cannot catch a treat if his life depended on it and as quick as it hits the floor it's gone and he would go without.
To ensure equality, we recognize this weakness, and specifically hand him his treat.  We call his name and say "Chevy, this one is for you."
The rest of the dogs know that this one treat is his and do not attempt to take from my hand.

Now as I stated in other blogs, Chevy has a habit of hoarding, and from this we found that we could give him the mail or newspaper and he would proudly bring it into the house for us. None of the other dogs are interested in doing this and we have elected not to train them to do so.
This is simply "Chevy's domain, and is a way to compensate for his lack of ability to catch things in his mouth.

If you wish to train your dog to bring in the mail, determine the size of your dog and whether or not he/she can handle the task.
Chevy cannot handle the full paper as it is too big for him so we use an insert instead.
He sometimes tries to tear the mail so we ensure that he only gets junk mail and bills that are "past due" because we would not want him to tear up a check or something that was important.

All training is based on a couple of principles.  Is the dog prone towards the feat, do you have the time and patience to work the dog, and finally repetition.

To get the dog to sit requires a few treats and repetition.  Start by placing your hand on back by his tail and simply and firmly say "Sit" and push down on his back until he sits.
When he/she accomplishes this, reward him/her with a simple statement of "Good Boy or Girl".
Repeat without rewarding the dog with the treat but only with the "Good dog" statement.
Do this a few times and finish it with the treat.
Do this for a few tries during the day and then reward him/her with the treat.
Once you no longer need to place your hand on his/her back and he/she responds to the command "Sit" you are halfway done.
Now that you have a free hand, you can use it to use a hand gesture that the dog will relate to as the "Sit" command.   I hold my hand up over my head for the sit command because if I am out in a field or across a busy street I want them to be able to see the command from a distance and sit until it is either safe for them to cross the street or me to cross over to them.
Take your time.  Be patient.  Do not try to confuse the dog by training them with too much information.
Master one trick a week or even longer.  It will depend on the dog.  Some love treats and can do multiples in a matter of seconds while others take longer. My dogs automatically sit, put out their paw and will sit pretty(beg) in a matter of seconds if they think there is a reward in form of a treat.
All dogs want to please their master.  Keep that in mind when a pup is overexcited.  It is not that they are bad, but rather so excited to please you.