How to get to San Blas
Puerto Vallarta to San Blas
Most people fly into Puerto Vallarta.  There are 4 buses daily from Puerto Vallarta to San Blas (7am, 10am, 12:30 and 3pm - subject to change)*. The trip to San Blas takes 3 1/2 hours. There are no bathrooms on these buses - nor do they make stops. The bus station is located about a mile from the airport.  Another option is to take a bus to Las Varas and a taxi from there.
To save money on the taxi, go out of the airport and walk over the bridge across the street before taking the taxi. (A taxi from here costs $12 to town, to the bus station much less, maybe $4).   You can hire a taxi from Puerto Vallarta to San Blas for about $120 dollars. By the way, if you get hungry there is a very good place for marlin burritos over the bridge from the airport. If you have to spend the night in Vallarta, the Aztec Hotel on Calle Madero is good - about $35 a night for a double and there are many good cheap restaurants in that area. There is also a hotel across from the bus station with clean but small rooms (Hotel Terminal) There is an hours time difference between San Blas and Puerto vallarta - PV is one hour later than San Blas. This is very inportant to know for your return trip.
*Buses from San Blas to Puerto Vallarta leave at 7:30, 10, 1:30 and 4:30.

Tijuana to Tepic
If you live in San Diego you can fly Volaris from Tijuana to Tepic.  Once in Tepic you can take a taxi to the bus stations - but the last bus leaves for San Blas at 6:45 in the evening. A taxi from the airport to San Blas costs $55 USD.  Tepic Airport is about an hour and a half from San Blas. When flying into Tijuana, you can walk across the Cross Border Express. There are not lines and you can rent a car or have a friend pick you up on the California side. They also have a shuttle to the trolley station at San Ysidro.

By Bus
You can take a bus from Tijuana for about $50 to $90USD.  It takes 33 hours (although they will always tell you it is 24!).  Except for the loud violent movies, the trip is not bad because they stop a lot for food and exercise.  Buy a ticket for Tepic, but ask the bus driver to let you off at the 'crucero de San Blas' and catch another bus into town.  Last bus at the crossroads goes by around 7:30 in the evening.

San Diego Border Information
If you are going by bus, or flying from Tijuana, this info might be helpful. On the American side in San Ysidro, there is a trolley right accross the border that can take you downtown - there is a shuttle bus from the trolly to the airport from Santa Fe Depot (train station) downtown. The trolley runs about every 15 minutes from early morning to late at night. Going into Mexico you first enter a no mans land - right after you go through the gates there is a place to get a tourist visa on the left. You can use the Cross Border Express to walk across the border from the TJ airport to the California side and visa versa. You need a boarding pass to do this.

Mazatlan to San Blas
They do not have a direct bus from Mazatlan to San Blas anymore. You should take the bus to Tepic and get off at the 'crusero de San Blas'. Trip takes about 5 hours. Last bus at the crusero runs around 7:30PM.

Guadalajara to San Blas
Take a bus to Tepic (3 hours), then change in Tepic for San Blas (1 hours 15 sminutes). It is best to take the express bus to Tepic - the one that goes through Ixlan takes quite a bit longer. (The last bus from Tepic to San Blas runs about 7:30PM).