Almost goes without saying, but never leave the house without a plentiful supply of poo bags. You will find that after a while every single handbag, coat or jacket you own will have its own little supply. 
Handy also to have a stash in the car. Not so much etiquette, but we also keep wet wipes in the car, a spare towel (in a deluded attempt to keep them less dirty), bottled water and a bowl for drinking, and a few spare treats. We also keep a spare collar and lead as we've met lost dogs once or twice while we've been out and it's always handy to catch them with as well as being handy to have your own spare.
Other points to remember are that although it's nice to be sociable with other dog walkers, the point of the exercise is to keep an eye on your dog at all times and not to be distracted by an interesting conversation or a mobile phone chat. Keep calls brief and keep your eyes peeled. Labs are notorious hoovers, and since so many park users (especially in the summer) are messy sods, you will often see discarded food items. These can be dangerous (e.g. grapes, cooked chicken portions) so it's important you stay aware of potential hazards.
Letting your dog chase or bark at cyclists, runners or horses is a complete no no.
Also a no is your dog invading a picnic, stealing food from a child. Generally, it's better not to walk your dogs in graveyards, war memorials and inside fenced children's play areas - these are often dog ban areas anyway but be sensitive. Find out what if any Dog Control Orders/dog bans are active in your area and dog policies in nearby Forestry commission areas etc. Be aware of the importance of not letting dogs off lead around livestock and during bird nesting season if you walk in countryside.
Jumping up at either children or any other pedestrian is a no. You may think (in fact, know ) that your dog is the most adorable creature in the world but that does not mean that other people should be bounced on, have muddy pawprints over clean clothes, or be knocked over (especially for elderly/frail people).
If other dog owners have their dogs on a lead, and if they take avoiding action when they see you , it is not personal. If you see any dog owner giving you a wide berth, respect this and don't let your dog bound over to "make friends" - this will not be the phrase the other dog owner will be muttering under their breath. This is doubled if the other dog is wearing a muzzle - dogs wear these for a reason - no need to be paranoid, but again, give a wide berth until you know differently.
Generally, you'll establish a pattern of dog walking at a certain time and favourite places and you'll quickly get to know other "regulars" and their dogs. This can be very helpful and you will probably make some wonderful friends. However, if you find the place you go is too crowded, unpleasant for any reason or just downright dodgy then don't hesitate to go somewhere else. Your walk is meant to be (and often will be) the highlight of the day for both you and your dog - life is too short to be stressed, threatened and miserable.