For serious cosplay, you need to look like the character, do lots of research about the character's background, their personality, their habits; and then try and become the character and embody their persona. There's no point dressing up as a character you don't look like, and then not acting like them.
Also, when making the cosplay outfit, attention to detail is key. Try and use the same clothing materials, colours and accessories as the character you want to cosplay. Wigs and contact lenses also help if you don't have the right hair or eyes.
However, if cosplay is for fun, then have fun. It doesn't matter if you don't look exactly like them...just have fun being that character for a day.^_^ Sure, if it's a cosplay competition, then go all out serious; but if it's for a con, then just have fun and mess around. I think being too serious about cosplay sometimes spoils it.
