Right, so the challenge is pretty much the title. When Britain enters the colonial game, have it try and keep the colonies in some what organised. They can try and wrestle the colonies into line if required, but in essence have the various communities have a representative on a council/proto-Parliament that governs under the Crown. This means one code of law, at most one governor/appointee to represent the monarch.
Bonus Points if
1) It happens before 1700.
2) No army needs to come and stomp the colonies
3) The Parliament begins to accept local native communities as members with representatives
4) It doesn't consider itself something other than British
5) It doesn't rebel
Super Bonus Points that are completely unrelated to above
6) Slavery is never accepted as legal.
The problem is that as long as Parliament's in charge, you're going to have issues with representation -- either the colonists won't get much/any, which will annoy them, or their MPs will end up swamping Britain proper's as demographics shift in favour of the colonies. The best solution would probably be to have the monarch retain more real power (probably in the fields of war-waging and foreign policy, which would also most likely be the main competences of any central imperial government); the colonial legislatures could then maintain ambassadors in the UK to make the King aware of any issues in the colonies that he needs to fix, make sure colonial interests are represented, etc. Maybe also have each colony liable to maintain a certain number of regiments and ships, based on their wealth and population, or else make them pay a sum (perhaps a certain percentage of their GDP) to a common imperial defence fund. Avoid imposing direct taxation, as this would inevitably lead to demands for representation, in turn leading to the problems outlined in the first sentence.
ETA: As for your bonus points:
(1) Not sure if the colonies were large/important enough yet before 1700 for Britain to really care all that much about how they're administered. If they aren't, maybe push the start date of large-scale British colonisation forward a bit (IOTL I don't think it started in earnest until the Jacobean period) so that the colonies are bigger and more important.
(2) If the British government doesn't overplay its hand and try and interfere too much in the day-to-day running of the colonies, and if it doesn't try and screw the colonies over for the benefit of the motherland (earlier *Adam Smith, perhaps?), the colonies have no reason to rebel and good reason to stay (being able to rely on the rest of the empire to help defend them from potential enemies). Hence, no need to keep them down with military force.
(3) Unlikely to work, for reasons outlined above, although you might be able to set up some sort of imperial council to make it easier for colonies to represent their interests to the Crown.
(4) The US colonists considered themselves British until the Revolution (and indeed, part of their legal case for independence was that the Crown was abrogating their rights as Englishmen). So, this shouldn't be impossible.
(5) See point (2), above.
(6) Erm, let's see... there was some anti-slavery sentiment around (there had been Papal Bulls condemning the Spanish and Portuguese slave trades as early as the 15th century), but the institution was so profitable that there was a huge vested interest in keeping it going. Maybe have abolition become more popular earlier, when the slave trade wasn't yet as big a business as it would later become and you could potentially nip it in the bud?