When talking about sharing the Gospel with others, "witness" and "testimony" are two terms that are used quite often. Along with this, these two terms also apply to court - and they are also more or less synonymous there as well. But in thinking about court and witnesses and testimonies; I also think that there is almost always brought into a trial "character witnesses".
Character witnesses don't testify to the case, but to the credibility (either for the positive or the negative) of those who have testified about the case. And this type of testimony is very important to the ones making the final verdict, because your life says a lot about your trustworthiness.
So taking this thought back to sharing the Gospel with others; what would a character witness say about us? Does our life say "yes, he/she speaks the truth" or "no, don't believe him/her"? Does it make the message any less true when our life is that of a hypocrite? No, truth stands as truth - but it's believability is compromised in the minds of those who need that truth. Our lives can actually be a stumbling block, keeping people from accepting the message of salvation.
I think every Christian should understand that we are obligated, commanded, required, to preach the Gospel (this is one of the main messages on my blog); but I would agree with Charles Spurgeon (a famous preacher) who said if you live the life of a hypocrite, before you speak about God, you should go 2 miles outside of your town - then when you go to open your mouth, you should say nothing.
The solution however, is not in following Spurgeon's advice; it is to stop living the life of a hypocrite - then open your mouth and proclaim Jesus; living in obedience. Let people reject God because they reject His message; not because they reject us and the message of our character.
1 Timothy 3:7
Moreover he must have a good report of them which are without; lest he fall into reproach and the snare of the devil.