look around the forum for threads on the subject (alot more of them than you think) and go to the "shipping yourself and the bike" section in the left bar.
forget ecuador and venezuela. Cartagena, Colombia is port of choice. Colombia is also very nice and you would be sad to miss it I am sure.
otherwise don't worry about it. so many people do it that you can easily find a boat in Panama city (contact the hostels that are mentioned in LP, they make big bucks selling passages and know all the boats)
I saw quite a few adv riders when I arrived in Panama city and they had already found a ship.
lastly I advice you to take the Stahlratte (google it) if you can. good outfit.
and last lasty I say, don't worry it is simple and easy. I was worried as well, but without reason.
South East Asia, USA, Central and South America and Scandinavia.