Is this for character pixels or battlers?
If it is for characters then you can look at what other people have made and alter them, there are also some character generators out there that you could use.
If it is for battlers however, think about the time era, cliche's, their alignment, profession and how these would infulence them
For example a knight would be in armour reflecting who he is affiliated to. A ninja would be in black and dark blue's with tight fitting clothes, a villan could wear a flowing cape a cowl or a mask, someone from the future would wear something shiny like chrome maybe with a wrist readout and blinking lights, while a caveman would wear furs according to seasonal changes.
Hope this helps you out.