I agree with many of those comments already made. Regardless of what "religion" we discuss, the living a life that reflects the tenets of your personal faith is the best testament. I have known many people in my life who have turned aside from Christianity because of the ugly actions of some "Christians." It's hard to believe a faith has value if the behaviors of those associated with it are ugly and negative.
Regardless of ones religious preferences, there is no better way to spread the faith than by living according to its precepts. I do not mean by this to "advertise," or wear ones religion on ones sleeve; however if one is a Christian, one should live by the teachings of Jesus, if Muslim, by the tenets of Islam. A famous poem by Edgar A. Guest, reads as follows:
One good man teaches many,
men believe what they behold;
One deed of kindness noticed
is worth forty that are told.
Who stands with men of honor
learns to hold his honor dear,
For right living speaks a language
which to every one is clear.
Though an able speaker charms me
with his eloquence, I say,
I'd rather see a sermon
than to hear one, any day.
One spreads the faith by living the faith in an unobtrusive way.
I like the first answer. I too believe this is an intensely personal question. On that note, I will answer it for myself in the hope that my story might encourage you or at least spark some ideas.
I attend an evangelical Christian church. On a weekly basis, there is talk of "evangelism" and how to do it. I find that there seems to be much emphasis (in the Baptist denomination especially, but perhaps others as well) on mission work, money for missions, and "relational evangelism" which is basically sharing your faith with friends.
On thing I think is terribly overlooked however, is the capacity parents have for spreading faith. It is pretty widely accepted that the Muslim faith is one of the largest (if not the largest) world religions, and is rapidly growing. It could be argued that Muslims are not necessarily evangelism oriented. So why is this religion so huge? Big families. Muslim parents are raising children in a faith that is strong enough to cause those children to eventually adopt it for themselves and spread it to their own kids.
My current faith focus right now is on my family. My husband and I are raising two daughters. It is not my current aim to "convert" the world to Christianity, but raising my children to understand my faith (and hopefully choose it for themselves one day) is my current pursuit.
Surely this has to be something that you would answer on your own. Those of us who answer questions on here are in such different circumstances than you (we're old, have children, etc) that our answers would not make sense for you. We might not even be the same religion as you.
So think about this -- what have you done? Have you acted in ways that make people respect your religion? That is a way of spreading the faith -- to show through your actions that your religion is a religion of good people. Have you participated in church groups? Those can spread the faith by helping others (like with charity) or simply by helping your peers maintain their faith (by keeping them in contact with people who share their faith). Perhaps you have done some of these things or other similar things. These would be ways you might have helped to spread your faith.