Format your screenplay correctly. Some extra length might be due to a font other than Courier-12, multiple spaces between your scene headings and action, or dialogue indented more than 2.5 inches from the left margin or spanning less than 30 to 35 spaces per line. Use screenwriting software such as Final Draft or Movie Magic for automatic industry-standard formatting as you write.
Read your screenplay to assess the economy of your wording and make adjustments for succinctness wherever possible. Avoid phrasing like, "She went inside the room and looked all over and then left after seeing that no one was there," where, "She enters. Peers around. It's empty. She departs," will do. Nineteen words then become eight -- essential not only for trimming pages, but also for having action read as action rather than prose.
Read your screenplay again, creating a beat sheet in a separate document as you go. Number and summarize each scene succinctly -- preferably in one sentence. Don't exclude details essential to understanding the story as a whole.
Label the main story's major plot points on the beat sheet: plot point number one -- sets the story in motion; number two -- makes clear things won't go as planned; number three -- raises the stakes, no turning back; number four -- lowest emotional point, all seems lost; and number five -- resolution. These should remain untouched during trimming.
Label the plot points for any sub-plots. Use a different color for each sub-plot.
Cross out extraneous scenes on your beat sheet that don't contribute to the development of your main story or sub-plots. Any scenes that seemed cute, cool or impressive when writing them but are now irrelevant must go, no matter how well-written. Eliminate the corresponding scenes from your script.
Assess your sub-plots, if you still have too many pages after trimming extraneous scenes. Decide which sub-plot is least essential to your script's impact or theme -- or which would least disrupt the story if eliminated. If you have trouble deciding, have one or more people read your draft and tell you which storyline they would miss least. Eliminate the most commonly rejected storyline. Cross all related scenes off of your beat sheet, then eliminate all corresponding scenes from your script. Delete additional storylines and scenes in this manner until you've reached a suitable page count.
Read your screenplay again to assess continuity of the remaining story. Make note of any missing information, or other issues due to your deletions, that could hinder readers from getting all the information needed to fully understand the story as it unfolds. Incorporate the necessary adjustments naturally and fluidly.