The key to curbing aggressive behavior is active training and prevention.  Instead of hoping your dog will be comfortable meeting new people and new situations, set up positive associations in order to prevent potential future problems.

• Socialize your dog with new people slowly and positively.  Don't put him/her in situations where they are not comfortable as negative associations can form quickly and last a lifetime.  Have new people give your dog treats (have them toss treats if your dog is really shy) or throw your dog a ball.  Teach them that good things come from new people.
• Socialize your dog with new dogs slowly and positively and often.  You may start by setting up play dates with a friend's dog whom you know enjoys other dogs.  If your dog enjoys the interaction with the other dog, your next step may be to include others that your dog does not know yet.  As your dog continues to enjoy the company of others, you may increase the amount of dogs your dog meets.  Please note that some dogs like one on one interactions better than large groups of dogs.  Make sure to praise all good interactions your dog has with other dogs and remove your dog from situations that are negative. 
• Just because your dog liked other dogs as a puppy, does not mean they will always enjoy the company of other dogs as the mature.  Socialization is an ongoing process.
• If you meet a dog on a walk (both dogs will probably be leashed), first ask the owner if your dogs can meet.  If the owner obliges, have the dogs meet with nice, loose leashes as restraint can increase the potential for aggression.  Praise and treat your dog for calmly meeting this new friend and continue on your walk.
• Give your dog dessert.  While your dog is eating her dinner, drop in a few bits of really tasty food.  This conditions your dog to think that reaching towards her food bowl is a very good thing.  Note: if your dog growls or nips when you come near the food bowl, a professional trainer may be needed to help with food bowl resource guarding issues.
• Teach your puppy bite inhibition.  In other words, teach your pup that it's not okay for him to put his mouth on any part of your body.  If your pup mouths you, make an "arp!" sound (this means "ouch!") and quickly redirect him to a special toy.  Also make sure the games you play with him do not encourage him to use his mouth on you (see play section).
• Teach your dog that it's okay for you to take her toys away.  This can be done by teaching your dog "drop it."  You can do this by making trades.  Let's say your dog has his ball and you'd like to play fetch with him.  You take a treat out of your pocket and wave it in front of his nose.  He drops the ball and just as he does, you say "drop it" and give him the treat and throw the ball.
• Be a calm and benevolent leader to your dog.  Make rules and be consistent.  Take an obedience class that promotes positive training and exercises.