By plane
Piarco International Airport is about 45 minutes from the center of Port of Spain. Allow more when flying out if your trip coincides with the evening rush hour as many people commute from the area around and beyond the airport to downtown Port of Spain.
The airport is served by:
American Airlines
Caribbean Airlines
British Airways
Liat
Continental Airlines
Delta
Copa Air
...and several others.
From Tobago there are 14 flights a day on Tobago Express, which is operated by Caribbean Airlines. Flights are on a De Havilland Dash 8, cost US$24 each way and last about 20 minutes. Despite the great frequency of flights these can get booked out very easily.
By ship
There is a Wednesday ferry from Venezuela. See Trinidad.
From Scarborough, Tobago there are hydrofoils that take 2 1/2 hours and conventional ferries that take 5 1/2 hours to Port of Spain.