Don't overcook it! Zucchini is a vegetable that turns mushy/soggy when overcooked. It has a ton of moisture/water content which will release as it cooks. If too much water releases, your zucchini will steam instead of sauté resulting in soggy zucchini. Keep an eye on it, work in batches if you have to (although it's not necessary for them to be put in a single layer) and stir frequently to avoid sogginess. Also, don't add salt too soon as it will draw out excess moisture and turn the zucchini mushy/soggy instead of crisp and tender.