The best thing the EU could do is get Acting President Turchynov on a plane to Brussels as soon as possible and sign the Association Agreement. The EU has shown itself over the past 3 months to be an utterly incompetent foreign policy actor. The extra powers won at Lisbon look ill-deserved, the posts of Foreign Policy Representative and EU President worthy of abolition at the next treaty change. However, the EU still has a massive tool at its disposal. The whole point of the Kremlin’s crusade all along has been to prevent the signing of the AA and to ensure Ukraine was on the path to the Eurasian Union, so signing the agreement would be a blow to Putin in no uncertain terms. It might even stabilise the situation across Ukraine and give opponents of the occupiers in Crimea a concrete reason to hang on to their Ukrainian passports. The Kremlin delights in knowing the west will never act so quickly and decisively. Oh that we could.
Close the Bosphorous to Russian shipping
Turkey has a role to play here, and a right to do so on a number of counts. Firstly, the Crimean Tatars, whose future inside a Russia with an awful minority rights record looks potentially appalling, are the Turks’ ethnic kin (as place names such as Kazantip or Bağçasaray bear clear testament to), so arguably Turkey has a moral obligation here, and the Crimean Tatar diaspora in Turkey itself would support this. Turkey's media is already claiming, rather in the way that Spain does with Gibraltar, that if Crimea breaks away, it should legally revert to Turkish rule. Although that's practically impossible, it does help to legtimise Turkish involvement. Secondly, Putin’s support for Assad has had direct consequences for Turkey, bringing with it massive refugee problems. Thirdly, Russia’s asserting itself on the Black Sea has major implications for the Black Sea security situation. Turkey should close off the Bosphorous to Russian shipping and insist on a place at the table if there is mediation. Or at the very least, Russian ships passing through the straits (there have been two in recent days) ought to be 'escorted' by their Turkish counterparts. One of two warships dispatched to the fringe of the region would also concentrate minds in Moscow. Erdoğan would probably appreciate a distraction from domestic issues, including a corruption probe. They might not have a spotless record themselves on human rights, but at the moment Ukraine should take all the help it can get.
An obvious one, but it took a very long time indeed and many deaths before it really looked like becoming a reality for the orchestrators of the violent crackdown in Kiev. It has been claimed however, by Mikhail Saakashvili for example, that it was the crystalising of the threat of sanctions that finally lead to the collapse of Yanukovych's house of cards. Sanctions against Russian officials might also try the patience of many of Putin's backers. There is also the potential for broader economic sanctions, and with the Russian economy now on the ropes, their impact might be felt quickly.