Here's the annoying part about a stuffy nose: you can do everything "right" all day and still end up mouth-breathing the second your head hits the pillow.
Your nose gets stuffy because the blood vessels inside it swell up (usually from a cold, allergies, or general irritation) and mucus piles on top. Anything that calms that swelling or clears the mucus helps.
Saline
Not steam, not some fancy essential oil blend — plain saline spray or a rinse. It calms the swelling and flushes out the gunk, it's backed by real evidence, and you can use it as often as you like without it turning on you. If you only do one thing off this list, do this.

Decongestants — but use them carefully
Decongestant sprays work fast, no argument there. The catch: use them more than 2-3 days and you can end up with rebound congestion, where your nose gets worse than before once you stop. If you need something for longer, oral decongestants or antihistamines (if allergies are behind it) are the safer bet — ask a pharmacist which fits what's actually going on with you.

Why it gets worse the second you lie down
This is the bit that actually explains why nothing above fully solves it. Lying flat sends more blood to your sinuses, which is part of why congestion always seems to peak right as you're trying to sleep. There's also a mechanical piece: the nasal valve, the narrowest part of your entire nasal airway, sits right inside your nostrils, and it naturally collapses in a bit more once you're horizontal and your muscles relax. Nothing about treating the congestion itself changes that.
That's the gap nasal strips are actually built for. The Happy Sleep Nasal Strip sits across the bridge of your nose and holds that nasal valve open mechanically, all night — it's making sure your airway doesn't cave in once you lie down. Treat the congestion during the day with the stuff above, and let a strip handle the part that only shows up once you're asleep.
General information only, not medical advice. If congestion sticks around past 10 days, comes with a high fever, or keeps recurring, get it checked out - could be a sinus infection or something else worth ruling out.