Turkey is an excellent place to find a crew position. Several dozen boats will be arriving in Turkey from Indonesia in 4 - 6 months. Many of the participants in the Indonesia Rally which left Darwin 2 weeks ago will be shipping their boats from Singapore to Istanbul.
You can find most of these boats listed here: http://www.sailindonesia.net/home/home.php
You can also follow many of these boats via their blog. Many that I know use sailblogs located here:
One of the last stops when leaving the Med for the Caribbean are Gibraltar and the Canary Islands. The ARC Gran Canaria to St. Lucia Rally is a good place to start looking:
You are more likely to find a crew position prior to a major crossing than you will while a boat is Island hoping within the Med.
Also, once you arrive in the Americas, there is the Pacific Puddle Jump
which many boats heading to the South Pacific take part in and a large portion of the those boats will be looking for good crew. So once you get experience crossing the Atlantic, you shouldn't have any trouble finding a ride to Tahiti or Bora Bora.
While in Bora Bora, it is easy to find a crew position from the Bora Bora Yacht club:
Most everyone is interested in a little extra help crossing the Dangerous middle to Tonga.
Once in Tonga, most boat wait several weeks to a month for a good weather window to either Opua, New Zealand or Fiji. If you find your way to Vava'u, Tonga in time for the Tonga Regatta, you shouldn't have any trouble finding a ride to either New Zealand or Fiji.
Hope this info helps!