Seriously, I become an overly anxious soul during testimony meetings. The potentially-ever-painful monthly thank-timony meeting. And having these meetings coinciding with Fast Sunday doesn't help either.
We have been instructed what this meeting is for, and how to appropriately speak during it. We have also been instructed on the importance of brevity and conciseness when bearing testimony. 
Elder Jensen once told us (and I summarise): 
A testimony is not an exhortation. And it’s not a sermon or a talk. Don’t you be banging your fist and calling me to repentance. That just offends me and the spirit.
A testimony is not an experience. Sure you can share a short experience to illustrate your belief. But don’t be giving me the long-winded version of what you did that week.
A testimony is not an expression of gratitude or love. Are you shocked? Again, it may be appropriate to include some gratitude or love in there, but that is not the point of a testimony or the meeting.
A testimony is not a public confession. Enough said.
A testimony is not a long explanation of how you know, but rather what you know.
And yet, this is what I regularly hear:
1. I'd like to thank that mystery person for delivering cookies to my door when I really needed them. I like cookies.
2. I love this ward. You guys are great. My last ward wasn't nearly as friendly, but you guys are great. And I love my family - I don't tell them nearly as often as I should.
[Elder Bednar once said these comments make him silently squirm in his seat.]
3. I'm so grateful for the birds in my backyard. They are nice and chirpy, and remind me of the time ...
4. I really like Young Women's. Yeah, it's way fun. But I don't like getting up for seminary much.
5. This week I did this, and this; and then this happened; and then my grandson called and this happened; and then I planted a tree, and watched it grow. I like trees.
I remember being taught by my parents (now translated) that I should get up and say one or all of the following only.
I now pass this on to you.
When you share, bare, or even expose** your testimony to others, you should say one or all of the following only:
1. Your belief in God, our Heavenly Father. You could extend this to your belief in the Godhead - God, Jesus Christ, and the Holy Ghost.
2. Your belief in Jesus Christ and His atonement for us.
3. Your belief in Joseph Smith being a true prophet of God, and in his role in restoring Christ's church.
4. Your belief in the current-day prophet, leading and guiding Christ's church.
5. Your belief in the Book of Mormon, being scripture alongside the Bible.
Some also teach that a belief in Joseph Smith and the Book of Mormon is actually the same, allowing for a belief in the temple as number five on the list.
** I once heard a special someone get up and say: I don't just want to bare my testimony. I want to expose myself. No joke.
These five basic principles should be the foundation of your testimony, and what you say during testimony meeting. They are the foundation of a testimony. We should say the things which set us apart from others, the things we believe to be true. We can expand on other principles of the gospel we know to be true and our appreciation for them, e.g. your knowledge that Heavenly Father loves you and how you appreciate that love; your knowledge that Jesus Christ lives; your knowledge that tithing is a principle from God because you exercised it and now understand it.
This parental advice was echoed by Bruce R. McConkie, who wrote the three great truths that must be included in every valid testimony were:
1. That Jesus Christ is the Son of God and the Saviour of the world (D&C 46:13);
2. That Joseph Smith is the Prophet of God through whom the gospel was restored in this dispensation; and
3. That The Church of Jesus Christ of Latter-day Saints is the ‘only true and living church upon the face of the whole earth.’ (D&C 1:30). 
Note: I understand that testimony grows through experiences. Our knowledge of what is true will gradually increase as we exercise faith and practice what we know, but the pulpit is not the time and place to elaborate for 5-30 minutes. We do have other opportunities to share our testimony - usually during Relief Society meetings, Sunday School meetings, etc. If you don't have those opportunities to share your testimony-building experiences, ask for them. Or hold a Family Home Evening dedicated to testimonies.
The following are examples of how testimonies should sound like from the pulpit:
1. "I know the Book of Mormon is true."
2. "I believe in God. I know that President Monson is our prophet today."
3. "I know Jesus Christ died for me. I understand the atonement more fully from my recent scripture study, and I know it to be true."
4. "I know Joseph Smith was a prophet of God. I know he restored the fullness of the gospel, and that the church is the same as it was in the days of Christ. I also know we are led by a living prophet today, and that we are able to know of these things for ourselves."
Do I need to bring out the Testimony Glove? (Oooh! It's currently on sale ....)
Want to know what puts me on edge in Sacrament talks?
Read my post on what not to do when giving a talk in Sacrament meeting.
 First Presidency letter, May 2, 2002; see also M. Russell Ballard, “Pure Testimony,” Ensign, Nov. 2004, 40–43
 Jay E. Jensen, ‘Bearing Testimony’, Ensign, Oct. 2005
 Bruce R. McConkie, Mormon Doctrine, 2nd ed. , 785–86