Ukraine's president has no choice but to remember Crimea:
Speaking at a news conference on June 5, a day after warning of a "colossal threat" of the resumption of full-scale fighting with Russian-backed rebels in his annual state-of-the-nation address, Poroshenko said that "every day and every moment, we will do everything to return Crimea to Ukraine."
He vowed to continue working with Western nations to maintain sanctions imposed on Moscow over its annexation of the Black Sea peninsula in March 2014, which followed Russian troops deployments and a hastily-staged referendum condemned as illegitimate by about 100 UN member states.
Not relinquishing their legal claim on the territory is just the first thing Ukraine needs to do.
Ukraine needs to fortify the outlets from Crimea to pen the Russians in to protect their southern flank in order to focus on the eastern and northern fronts; and to protect Ukrainian artillery assets that could bombard Russian defenders.
Ukraine also needs to build up their long-range missile arsenal in order to bombard Russia's Crimea bases should Russia go to war against Ukraine. This would hold the Sevastopol bases hostage counting on Russia believing they are a big asset for power projection into the eastern Mediterranean Sea.
Then build up mine laying assets in order to hinder Russia's use of the naval base in case of war.
Finally, Ukraine should bill Russia for the conquest of Crimea, its property, and its natural resources; and charge a monthly rent until Ukraine regains control. And then pursue these monetary claims through whatever legal channels are appropriate.
I imagine that any Western company that deals with any Crimean entity could be challenged in various national court for not contracting with the legal owner of Crimea, which is Ukraine.
Perhaps part of the money damages recovered this way could go into a fund for Crimea that pays for a government-in-exile and pays for losses of any residents of Crimea who fled--or flees in the future--to Ukraine rather than live under Russian control; and which also earmarks money for a trust fund for reconstruction in Crimea once it returns to Ukraine's control.
Russian claims defy repeated Russian relinquishment of the territory to Ukraine, recall, despite Putin's claims. Ukraine must challenge Russian control of Crimea at every turn to damage its value to Russia now and prepare to put it at risk as an asset for Russia's military until Ukraine can get it back.
UPDATE: Let Russia isolate themselves when they insist on justifying their control of Ukrainian territory:
Russia has refused to sign off on the U.N. nuclear watchdog's annual report because the document lists a nuclear reactor near Crimea's Sevastopol as being in Ukraine, two diplomats present at the meeting told Reuters on Monday.
The report goes out. Just without Russia's signature.
We should look for ways to make Russia stand apart from the rest of the world over Ukraine.