There isn't a simple answer nor a set formula to solve the problems of the unfriendly dog, but all dogs tend to conform to predictable rules of behaviour.  

Dogs tend to defend their territory from humans and other dogs, regarding both as its own kind.  The territory may be the dog's kennel, or that of the pack - its owner's house.

Your objective will be to confuse the dog by not rewarding it by your actions.  The dog's intentions are to make you leave its territory by showing aggression towards you.  You naturally leave as quickly as possible, thus rewarding the dog and encouraging it by its success.

1. Even friendly dogs will bark at you, as it is their job to defend their territory.  Talk to the dog in a firm but quiet voice and if it approaches in a friendly way - no snarl or hackles up - stand still so that it can sniff you.  Many dogs will respond to your greeting by wagging their tail.  Others will ignore you but do not be too forward or over-friendly.

2. As you approach the dog look at how it reacts to you.  If it stays put or backs off, it probably regards you as a dominant intruder and so will be too scared to attack.  If it walks or runs towards you with its tail wagging in a low position it is probably friendly and will not bite.  If it stiffens up, holds its tail high, snarls and stares at you, then be on your guard.  If it shows its teeth it may be safest to go no further, but don't turn and run - back away slowly.

3. Never run past a strange dog, or walk quickly away from it.  This may make it chase you and you could get bitten as a result.  Always walk or back away slowly, facing the dog, if you feel it may chase you.

4. Try to avoid showing any fear.  A dog can read fear in your eyes and body movements.  Keep calm, whistle, walk slowly or speak firmly to the dog.  Dogs rarely attack so don't be over anxious about every dog you meet.  Most are scared to get into a fight, but like to act tough on their own territory.  Don't ignore dogs, but behave in a friendly and confident way at all times.  You will lessen their own fear.

5. Never stare at a dog.  Staring is a threat - a dog may read it as a challenge and attack you.

6. Remember, a dog can tell if you like them and a dog biscuit in your pocket is a better insurance than a stick or other form of repellent.  However, do not feel bad if you like dogs and they all respond by barking at you.  They are just doing their job, as you are doing yours.

7. Finally, don't forget that many delivery people or occasional visitors get bitten as they are leaving the house.  Remember the golden rule - if in doubt back away slowly, keeping the dog in your sight all the time.