I actually think that, in a roundabout way, Spain being weak made the Spanish colonies that declared independance weak, while the British being strong meant that revolutionary America had to be strong to break away. Granted, the U.S. had allot of French help, but they had to look like they had a chance to get it, and that in and of itself pushed them a long way towards stability, as did the early threat of being reconquered. One must remember that before the ARW, there were other independance movements that were strangled in their cradle (first to come to mind being Bacon's rebellion). Because of this, in a very Darwinist way, only the fittest revolutionary movements survived.
Meanwhile, weak Spain and casual British support for anyone willing to screw with Spain (as opposed to France looking long and hard at the U.S. before making a move) meant that comparatively less fit independance movements prevailed in Central and South America, and their isolation from most would be conquerors (especially Spain, who would be first on that list, but had no chance of interfering thanks to the British and their own weakness) allowed them to infight instead of uniting against outsiders. Of course, the native element of society undoubtedly would have made developing a unified identity more difficult in most of these places, but that doesn't change the fact that having the freedom to waste resources on infighting was a bad thing. I also know that this can't account for everything, Brazil in particular being an example of bad luck and wrong man at the wrong time, but I feel it can help account for the Spanish colonies.
So, keep Spain strong, and keep their Atlantic Navy strong enough to not be walked all over, and the colonies will have a difficult time breaking away, so that the ones that do will need to be very strong and united themselves to get away with it.
This is an interesting way of seing it. However, I am not fully convinced. Yes, the British were willing to support anyone against Spain... before 1808. Afterwards, that is, in the very moment South American uprising occured (around 1810) the British were allied with those in Spain who fought against the French occupation, and weren't willing to openly support South American indepentent movements. That's why some Southamerican leaders like Bernardino Rivadavia, who had every reason to like Great Britain (they saw Spain and backwards, admire the British system and were pro free trade) ended up becoming very mistrusful vis a vis the British, and their empty promises of support. Britain didn't recognized Argentina's independence, for example, till 1825, way after the war had been won. And its support wasn't significant, even though Argentina becoming independent was in Britains best interests.
Where British support was decisive was in frustrating the efforts of the Holy Alliance to lunch collective expeditions to the Americas (Russian offered the Spanish help to recover their empire, for example). But this doesn't mean Spanish American revolutionaries had it easy. As in the case of the US, previous indepence movements had failed. Even Bolivar failed several times, and the war in Venezuela was quite brutal.
Of course, maybe if there was a serious threat of re-anexation by a strong Spain or by other power, Bolivar's calls for a Spanish American confederacy might have gained more support