When asking a girl out, is there something you should avoid to not make yourself look desperate?

Yeah. Don't come off desperate or needy. Just be confidant and ask her out like you are giving her the opportunity of a life time. Don't stutter, beg, whine, confess any deep feelings you may have for her. Be Dom de la Nooch.

Asking repeatedly is a sign of desperation.

It makes it seem like if she says no, your whole life will be ruined.

Unless she's being asked by fifty million guys at once, you only have to ask once.