Getting an engagement session is a really interesting idea, and not only because you'll get to have beautiful portraits that you might end up using as decoration on your reception room, but because you'll get to interact with your photographer, as well as learn what to do, and what not to do, when it comes to posing.
You need to remember that in order to get beautiful pictures; you need to understand that there are so many factors to take in consideration.
1.- Your photographer's skills are of course one of the most important things to get in consideration; but also, the chemistry between you and him/her, will play a special role on weather you love, like or hate the final results.
2.- Your background; you need to know that your background will be a good 25% of your picture, or depending on the style; it might be even more.
Try to find a place where you feel comfortable; you will be posing, and if you are more on the shy side; It might be difficult for you to create this romantic vibe when hundreds of people are passing by.
3.- Props; always try to include a prop or two; this will help you find a story to tell; either creating a picnic, or cooking a cake, all of this will create a story that will make the session way easier.
4.-Clothes; try to match your outfits; you can match colors, textures or styles; but try to make it in a casual way; do not force it, or you'll end up looking like a mariachi band of two.
5.- Have fun, love each other, if you make your engagement session playful, it will reflect way better your feelings for each other; this love and connection between you and your fiance; will be a definite wow factor to add to the picture.