The San Francisco Chronicle had an article a few months back (or maybe a year or now, I forget) about traveling to Cuba. They told you exactly how to do it. You can fly direct from Mexico City or Panama City (the two I remember off the top of my head) and they said that Cuba wont even stamp your passport. They (Cuba) know the drill and just want visitors to spend cash. They also suggested exchanging US dollars for Euros or something before hand to avoid the "crime" of spending US currency in Cuba.
If my level of Spanish wasn't just below "crappy" I would be hopping a direct flight to Panama City for a long "weekend".