For something "impressive," you're trying to draw looks from the crowd, regardless of what outfit you're wearing. To get that effect out of any costume, a lot of that has to do with how it's sewn or put together, and the type of fabrics used. Try to having clean seam lines and well-fitting garments made with a sturdy fabric. Fabrics can include, but are not limited to: bottomweights/sportswear (twill, cottons, poplin), denims, canvas, matte satin (casa satin) and the like! By using these fabrics your costume will have a much sturdier appearance — they are also easier to clean and iron as well. Couple that with a well-fitting garment (keep it from hanging funny, or falling down accidentally and such), and you'll be the hit in the crowd! Also your costume will most likely be washable, and last for many wearings.
For those interested in slightly "easier" cosplay (if you're not as confident in your sewing), I always suggest finding a character who tends to wear modern clothing. You can easily get a lot of garments from your local thrift stores that can be used to create a whole host of outfits! Think along the lines of some of your favorite television characters. Top it off with a character wig, and you'll be good to go!
For a fun take on a costume, a poodle skirt can help. Make a poodle skirt out of felt — just a big circle skirt — in the main color of the character you're going to be. Then draw, paint, or glue on a design or emblem near the bottom of the skirt that represents something about that character. For example, Link would have a Triforce design, Sailor Moon a crescent moon and Captain America a shield design. Pair the skirt with a plain colored shirt, a neck scarf in a coordinating color, and toss on a character wig and you're ready to go. It's comfy and fun! You can always do an interpretation of a character, such as a "modern day version" (make it out of clothing you can find in thrift stores in the character's colors) or zombie version (tear up/wear out your garments and maybe add some fake blood).
Are there a few good cosplay hacks you recommend to create a memorable style?
1) Never make a bowtie you have to re-tie all the time! There are ways to make big and small bowties that look just right by using simple rectangles, and there's no tying involved. Just sew them to your costume, and you're ready to go! (there's a great tutorial in Epic Cosplay Costumes that covers this very process in-depth!)
2) Don't reinvent the wheel — you don't have to make everything! Use regular, every day garment pieces to accent your costumes. Things like socks, T-shirts, dress shirts, jeans, and the like. Reuse parts for multiple costumes!
3) Fabric Paint — by mixing $2 craft paint with Textile/Fabric Medium — all found in the craft paint areas at craft stores — you can make an inexpensive, but durable paint that sticks to fabric to help you make the details you want!