To expand on the Rippetoe bit:

" The warmup sets serve only to prepare the lifter for the work sets; they should never interfere with the work sets. As such they should be planned with this in mind. The last warmup set before the work set should never be so heavy that it interferes with the work set, but heavy enough that it allows the lifter to feel a heavier weight before he does the work sets. It might only consist of one or two reps even though the work sets are five or more reps.

– Mark Rippetoe

The warm-up is important not only to prepare the muscles for the impending maximal load, it's also to get some extra technique practice in. This becomes especially important with Squats, Deadlifts and Power Cleans, where technique deteriorates as weight increases.

Here is Rips' warm-up template (weight x reps x sets):

Originally Posted by Mark Rippetoe

As a general rule, it is best to start with the empty bar (45 lbs.), determine the work set or sets, and then divide the difference between them into even increments. Some examples are provided in figure 5." (pg. 196)

Squat

45 x 5 x 2

95 x 5 x 1

135 x 3 x 1

185 x 2 x 1

225 x 5 x 3 <--Work Sets

Bench Press

45 x 5 x 2

85 x 5 x 1

125 x 3 x 1

155 x 2 x 1

175 x 5 x 3 <--Work Sets

Deadlift

135 x 5 x 2

185 x 5 x 1

225 x 3 x 1

275 x 2 x 1

315 x 5 x 1 <--Work Set

Press

45 x 5 x 2

75 x 5 x 1

95 x 3 x 1

115 x 2 x 1

135 x 5 x 3 <--Work Sets

Power Clean

45 x 5 x 2

75 x 5 x 1

95 x 3 x 1

115 x 2 x 1

135 x 3 x 5<--Work Sets

Sets 1-5 are to warm up, sets 6, 7 and 8 are the worksets. With all exercises (with the possible exception of the Deadlift) the first two sets are warmed-up with an empty barbell for 2 sets of 5, and the following 3 sets are incremental percentages of the workset (ie. 40%-60%-80%). You can rest as much or as little in between warm-up sets as you desire but I usually find the time it takes to swap out the weights is adequate.

Note that in all cases, as you get closer to the actual working weight, you do less reps in your warmups. The idea is to get the feel of progressively heavier weights in the hands/across the back prior to beginning your maximum weight sets.

I hate doing math, but I also like to be precise, eventually my hate for math won out and so I designed a Starting Strength Warm-Up Calculator. My calculator attempts to remain faithful to Rip's warm-up template, although I took some liberties with the deadlift. On deadlifts I lowered the warm-up sets/volume because 1) Those muscles were already warmed from the Squats 2) It became a lot of volume once you hit the heavier weights.

I offer the calculator in 2 formats: http://sites.google.com/site/startingstrength/Home/StartingStrengthWarm-UpCalculator.xls?attredirects=0"

Taken directly from here:

FAQ:The Program - Starting Strength Wiki