Assertiveness is a trait that can be developed for enrichment of personal and professional life. It's easiest to begin by identifying the wrong conditions. If you are in a situation which requires you to convince other people, avoid any and all of the following:

#1 Hiccup
#2 Foggy glasses
#3 Bleeding nose
#4 Dental anaesthesia
#5 Open shoelace
#6 Walking on Blitzeis

As each of these eventualities halves your assertiveness, having just three of them at the same time brings your potential to convince down to 12.5%. Such a cumulative effect is of course very unlikely.

(The list is by no means complete)