Avoid Disclaimers!
I have a cold...
I wasn't prepared...
I'm not having the greatest day so...
When you preface your speech with a disclaimer, you are telling your audience that you are nervous and unprepared. Once you tell them you may stumble, cough, or forget something, that is all they will think about. You are setting yourelf up for failure.
Skip the disclaimers, keep it to yourself, and push through! Stay positive mentally and verbally and your audience will view you favorably!