Research the publication before pitching
Sending an uninformed pitch is the fastest way to get rejected. Before pitching, read 20-30 recent articles in the publication. Understand their tone, typical length, the audiences they serve, what topics they have recently covered (so you do not pitch something they published last month), and whether they accept freelance pitches at all.
Many publications publish their submission guidelines online — find and follow them exactly. Guidelines tell you the preferred pitch format, word count, and submission email. Deviating from published guidelines signals that you did not do your homework.
An editor can tell in the first sentence of a pitch whether the writer has read the publication. Generic pitches that could be sent to any outlet are deleted immediately. Your research should be specific enough that you can reference a recent article, name the section you are pitching for, and explain why your idea fits their readership.