One of the keys to a good summary is making it short without making it vague. If you are a naturally wordier writer, you probably have techniques to trim down your word count inside your book already. Many of these techniques will help you write a concise plot summary too; however, I won't be covering all of this techniques in this post. Instead, I will be focusing on tricks that are specific to the plot summary format.
My first suggestion is to save old versions of your summary as you edit. When you write out the same ideas several different ways, you will likely have different strengths in different versions. You can't always connect all of your good lines and phrases together, but if you save the old versions of your summary, you can easily recycle those old ideas when they become usable again.
Next, make sure you know what the most essential information is. This will vary slightly by the type of summary you are writing, so make sure you are familiar with the expectations and goals of each type of summary. Usually, you will want to make sure that you've covered the protagonist's goal and the stakes. To make the plot feel cohesive, I recommend focusing on things that are resolved in the climax. If the inciting incident is complicated, focus on the parts that tie into the protagonist's goal and stakes. If you can explain the climax without mentioning other subplots, you will likely have found the most concise description of your plot.
If that still is taking too many words, you can trim down the goal and stakes to the most specific and emotional details. Avoid things that are vague, such as "save the world" or "find love. " These words don't carry their weight in so tight of a word count.
Character background and worldbuilding are often an easy place to cut down words. Only include enough background and worldbuilding to explain what the protagonist is trying to do and why. (Having trouble trimming down the explanations of your unique terminology? Check out this post. Have a lot of character's to juggle? Check out this post about naming your characters in plot summaries.)
The best way to pack a punch with a few words is to have those words multitask. When possible, use adjectives and nouns that carry implications to paint a larger picture. In the Harry Potter series, Harry learned that he was a wizard, not that he had magical powers. The word "wizard" implied castles, wands, and magic spells without an actual explanation about the kind of magic Harry could do. Try to do the same in your plot summaries. Use words with emotion and implications. "Tyrant" is more specific than "king." "Space ship" conveys more information than the word "vessel." "Fatal accident" tells readers more than "death." When you are finding yourself explaining lots of context, see if there is a more specific word that would carry more of that meaning on its own.
Paragraph and sentence structure can help you out as well. If you have an essential concept that is taking up more words than you can afford, sometimes you can trim the word count down by grouping all the sentences about that topic together. This helps them build off of each other and naturally provide the necessary context for readers to understand the more complicated or in-depth parts. When something was just mentioned in the previous sentence, you can say "this plan" instead of "their newest attempt at a prison break" or "the woman" instead of "the protagonist's brother's ex-girlfriend." By grouping sentences about these things next to each other, you remove the potential for confusion without adding more words.
Additionally, complex verb tenses (will be destroyed, had to be done, began to go) are often unnecessary in a summary. A simple past or present tense verb will usually work. When possible, say "left" instead of "started to set out on a journey."
Through these methods and a lot of time and patience, you should be able to summarize the essential information about your plot within the required limits of your summary. While you are editing, don't forget to make the summary enjoyable to read. Your author voice is one of the most important things you bring to the table when you're trying to get an agent or publisher.
Happy summarizing!