Mention the Metro Trip Planner to anyone who has experienced it and you most likely see their eyes roll back and a bit of a smirk develop. It's not that the trip planner is that bad; it's just that it's really not that good.
The trip planner will (most of the time) tell you how to get from point A to point B, but it rarely gives you the best route. However, if you use it as a tool and not the ultimate answer, it can come in very handy.
It helps to start by understanding the trip planner's limitations, so let's take a look some of the most important:
Knowing these limitations, we can work around them.
It does not consider options with more than two or three busses.
It favors Metro routes over other choices (e.g., Big Blue Bus or Foothill Transit).
It stops at the first workable solution it finds; in other words it does not continue until it finds the optimum solution.
Begin by entering the beginning and ending points for you destination, the time you wish to leave or arrive and your preferences. This will hopefully give you the first cut at your answer. If the trip planner tells you that you can't get there from here, try going to an intermediate point, C, and then from C to B.
The first cut will get you two important pieces of information:
1.) One bus line that serves your departure point.
2.) One bus line that serves your destination.
Now try running the trip planner, but going the other way. This may reveal another bus line you can use.
Use a third point, C, as described above. This can also help you find the alternative lines.
Look a the Bus Map to see if you have any other options.
Once you know which lines serve your departure and arrival points, you can check the schedules of the various lines. You may well discover an alternative that saves you time and/or money. (For an example, see the three different ways from LAX to Disneyland.)