Depending on day of week and time of day, the lines at the ends of the cable car line can be long. Just something to keep in mind when you plan.
If it's just one person, and you're willing to stand, go one stop after the beginning of the line. I used to date a guy on a cable car line, and it was always impossible to get on at Market and Powell. But I could always get on the first car that arrived at Market and O'Farrell.
This probably won't work with a family of six, but it works well for one person.