This is an attempt to answer the question.
Faith in America dies hard, and revives with every empty gesture, every new appointment, every rumour. Many analysts have developed sources of inside information and, of course, an abiding interest in the workings of insider politics. Their efforts naturally encourage placing great importance on at-the-top decision-making, but that inclination can mislead. The public, big-picture evidence on US policy decisively outweighs any stirring of hope that inside information may have provoked. It has done so all along. Several abiding themes stand out.
First. Where the US is concerned, one clear preoccupation never relents: preventing the spread of advanced anti-aircraft and anti-tank weapons. These weapons would pose a much greater threat to Israel than the big anti-missile systems that get attention such as the S-300. That's because Israel can easily destroy the big systems, but it cannot destroy the small ones. And like it or not, keeping Israel safe and happy will always come higher than helping Syrians on the US policy agenda.
Second. The US public does not support aid to the Syrian rebels and never will. Current US attitudes are shaped by the defeats or humiliations in Vietnam, Lebanon and Somalia, and the non-successes of Iraq and Afghanistan. These non-successes will look worse and worse as Iraq smoulders and Afghanistan collapses in the wake of a US pullout. It does not matter that the US would have to practically provide the muck itself to get into a Syrian quagmire. The public will always see a big quagmire risk and the politicians know it. Always, always, US domestic politics trump foreign policy, and domestic politics will always tell against arming the rebels.
Third. The US has no short-term or obvious vital interests in Syria. The uncertainties of a post-Assad future offer no assurance that the next régime will be more tractable than Assad, and what if it was? Israel wouldn't trust any Syrian régime nor fear it; there's no pressure from Israel to do anything. Yes there is oil and there are oil pipelines in the region, but oil's political importance has fallen off a cliff with the proliferation of new sources, not least within the US itself. Syria doesn't matter to the US.
The fourth obstacle to US aid derives in part from the first three. Given lack of public support, lack of vital interests and domestic dislike for 'foreign adventures', the US wouldn't dream of serious intervention in Syria without the UN stamp of approval. (UN approval, even if obtained by lies or manipulation, has always been important to the US in ventures outside its recognized spheres of influence.) But this, of course, will never come.
For these reasons the US will never deliver serious aid to the Syrian rebels. Statements and conferences suggesting otherwise are mere bluff designed to contain the ambitions of Russia and Iran. The US constantly finds fault with the Syrian opposition, not because it would help the opposition if these faults vanished, but because this is part of the bluff: Assad's supporters, hopefully, will see the US ready to step in just as soon as certain conditions are met. It's all posturing. There are good long-term reasons why US prestige and political credibility require supporting the revolution: the US will have little future in the Middle East otherwise. But these long-term reasons have no chance at all of prevailing over short-sighted obsessions.
Why does the US trouble itself at all about Syria, and help keep a small arms flow going? Because it does itself care about Iran, and wants to support its Gulf State allies who care very much indeed about Iran. But these allies are probably even more worried than the US about arms falling into the hands of Sunni radicals, so the help will remain modest. The US role here is nearly superfluous; the Gulf States would supply some arms with or without US involvement. And if the US thought its concern merited supplying advanced arms, it would have supplied them long ago. Even if the prolonged fighting spreads extremism, the US and its allies prefer poorly-armed extremists fighting in Syria to well-armed apparent moderates they don't trust.
The US position, then, is almost set in stone, and it affects the stance of the European powers. Only France and the UK have shown any interest in helping the Syrians. But France will not act without powerful support, not least because many observant French Catholics side with the régime. On top of that, the French Socialists have an unblemished record of cowardice and inertia, going back to the Algerian war. In the UK, the government must deal with popular sentiment, strongly against intervention, and has no intention of moving without at least the French on board. But neither government would do anything without at least NATO support, and that's not to be had without US commitment.
If all this is correct, the diplomatic and lobbying efforts of the Syrian opposition need reorientation. What follows are not specific suggestions; they merely try to indicate the priorities these efforts might require.
First, the US should be treated as a write-off. It doesn't matter what statements they issue; what conferences they call or attend; what officials they send where or what appointments are made. The most the US will do is discretely back others' efforts, an executive initiative that, whatever the legalities, in practice does not require Congressional approval. FSA attempts to look nice to the Americans are a waste of time. Even if I overestimate the uselessness of the Americans, writing them off is as likely to move them as trying to please them. Fear of losing influence is probably a stronger motivator than people begging you to exert it.
Second, and consequently, priority should go to getting help from Middle Eastern powers. I don't know enough about them to say how, but certainly the Gulf States and to a lesser extent Turkey are deeply concerned about Iran. This is the first time Iran has been involved in aggressive military action in the area. Presumably the Gulf States would regard their victory as a catastrophe.
Third, the best prospect for support outside the Middle East is France, which still has aspirations to matter in the Mediterranean. It would help if the FSA made some showy gestures towards Christians. Hollande's plummeting popularity might induce him to supply the rebels as a bid to get support of Sarkozy-style rightists.
Most important, perhaps, is a change of mindset. The US, for all its money and power, has deservedly become the laughing-stock of the Middle East. The EU has long been known for the fecklessness of its foreign policy. The UN, for the first time since the Cold War, is hopelessly deadlocked in the Security Council, and its officials shame themselves in their efforts to please both sides in the conflict. By comparison, there is change and therefore hope in the Middle East. However frustrating it may be to deal with the grey old men of the Arab League and the beleaguered Turkish government, that's where the focus needs to be. Western countries will never respond to the feeble direct pressure of Syrians anxious to please. Only a resistance dismissive of empty Western gestures has any hope of attracting genuinely useful support.