Have a look at the Cruising Associations web site. The often have skippers looking for crew. Like this one from Ian Griffiths:-
"I am taking Simbaroo our Beneteau 411 from the Uk (poole) to Croatia (Zadar) starting May 24th 2008. I hope to be somewhere in the Balearics probably Palma Majorca by 2nd week of July and am offering crew positions to take Simbaroo from there to Croatia via Sardinia, Scicilly, and Italy. The trip to take 2 - 3 weeks."