What you wear to your engagement session matters.
I come to you humbly as a guy who also enjoys wearing the most comfortable raggedy t-shirt he owns to most things (aside from weddings!) and I hate to tell you but while comfort is important... You can't show up in that jersey you bought ten years ago with the holes in it. You just cant, its not going to look good because your fiance is going to be glaringly upset with you in all of your photos. That doesn't mean you can't be comfortable though!
Confidence is key
Wear what makes you feel confident (within reason, those Looney Tunes pj's you wear to walmart should stay at home.) You can absolutely rock a formal look and be the best dressed in the area or wear khakis and a black t-shirt while your girl is rocking a dress. It's totally fine. One thing though: if hiking is involved, bring the heels but don't hike in them. Most couples wear proper gear to the location and change into the more formal look there when hiking on hills or moutains.
Please, don't wear matching outfits.
I'm not talking about wearing a black dress with his all black suit. That's totally fine! I'm talking specifically about completely matching outfits. We've all seen it and honestly it does not photograph well. My recommendation is to try complimenting colors & outfits. You can both be casual, but don't both wear the exact same sweatshirt and khakis. You are one couple, not one organism.
Simple is better.
This might sound kind of obvious but the focus of your shoot should 150% be you and your partner. Clothes without crazy graphics or logos on them will photograph better. Keeping those clothes simple and graphic free will absolutely serve you better photographically!
Maybe you grab a sign that says "I do" or has your wedding date on it. A blanket you can snuggle under or some cool lights also make for fun and unique photos.
Hair and Makeup
So you're going to spend all this time and investment on a photoshoot, but not your hair? No! You should totally have your hair done and get make up done by a pro because who doesn't enjoy a little pampering? You'll appreciate it!