As a rectangle, you have the advantage of lots of styles looking super flattering on you, you just need to focus on creating a defined waist and some gentle curves.
Your overall shape is well balanced however, defining your waist is important.
If you are relatively slender but you want to appear curvier, choose clothing that adds volume equally to both your upper and lower body.
A knee-length pencil skirt that tapers at the hem will help create curves in all the right places.
Choose straight lines and structured shapes which can be used to create more shape.
If you have this body shape and want to create some curves, your tops and sweaters should be adding width to your shoulders, bust and hips while at the same time pulling in at the waist. But you may want to accentuate your slender shape, in which case, you simply need to define your waist better. To create curves choose round necklines. To accentuate your slenderness go for narrow necklines such as funnel necklines, roll necks and turtle necks. Sleeves are a great way to add interest. You can choose balloon sleeves, or wide cuffed sleeves, just avoid sleeves that are fitted and shapeless as they will only accentuate your boxy shape. Relaxed fits with curved details will soften your silhouette. Have fun with bold patterns and textures too, just make sure that your tops end below the hip line, if they end at the waist they will make you look more rectangular.
The right trousers for you need to add curves and volume to your lower body. Choosing trousers with hip, thigh or leg details will create even more interest. Skirts are also a great way to add shape. Flared hemlines and pleats, bright and light colours will also help to create more volume. Wide leg, flowing trousers will be very flattering on you, but if you are a slender rectangle you could also try looser fitting “jogger” shapes to avoid swamping your frame. When it comes to jeans, you should choose washes, pockets and embellishments around the hip and bottom area. If you want to opt for skinnier fits, just make sure they are a tight fit around the hips to help create curves.
A great dress for a rectangle body is one that is balanced top and bottom put pulled in at the waist. Choose dresses that have a defined waist like wrap dresses or simply add a belt.
Pattern, colour and embellishments around your neckline and shoulders will add interest to your top half, but be careful to balance this out with a shapely skirt or trousers.
Tailored jackets and coats which are nipped in at the waist are also a great way to create a balanced silhouette.
For a more casual alternative , the pockets and detailing on a structured “western” style jacket will look great too.
Avoid shapeless garments with a boxy fit that hides your body shape.
Avoid frills, ruffles, double-breasted styles and clothes that add too much bulk.
Avoid creating any extra bulk around your waist by tucking things in.