1) Confidence - Everyone is staring at you when you give your speech. What you have to do is be confident. Be confident in your voice and your posture. Don't fidget or sway side to side. Stand up straight and in one place. Look relaxed. Feel relaxed. The thing you go to remember is that no one knows the stories that you are about to tell better than you do. You didn't just make this stuff up on the fly. This stuff actually happened and you actually mean everything you say. You are the freakin' source, end all, dictator of this stuff. If Bob got mauled by a bear and the groom screamed like a little girl, gosh darn it, he screamed like a little girl cuz you said so. Be the speech, be the topic, because no one is the wiser.
2) Don't say umm, hmm, like - This isn't the valley. Don't say like. Don't pretend like you are in the movie "Clueless". You don't look like Alicia Silverstone and you must learn how to parallel because not everywhere has valet. Umm and hmm is not a word. They are modifiers because you forgot what da heck you were going to say. If you must use them, please do it softly. If you are started going..."We met high school and our fifth grade teacher was ...UMMMMM...
", people are going to laugh. Ok, they aren't going to laugh, but you will definitely be the butt of some jokes later on. The best thing to do is just pause. Take a break and compose your thoughts and then continue on.
3) Don't ramble - This isn't English class or Speech 101 where you must speak for a designated amount of time. This your speech. You have as much time as you need. If it is over in 1 minute, then it just be one good ass speech. If you are over in 10 seconds, that better be the best one-liner ever. Don't let your speech go for 20 minutes and it is filled with nothingness. Fill your speech with substance.
4) There has got be a point - There should also be at least one good story. Generally you want it to be a story that is funny and embarrassing. If you have two stories, the 2nd one should contain both your subjects, i.e. Bride and Groom. But don't go on and on about the story. Make it short. Make it concise. Make it funny. No one wants to know that Jimmy's fifth grandmother was on insulin unless it is totally related to the story. The story has to have a point. It should be structure...Bob is a cool guy...
...that is why Bob is cool.
5) Preparation - don't forget to practice. The best stories come from those who memorize their speeches. Don't read off a piece of paper. Don't read off your iPhone. If you want to deliver jokes and want them to work, do it from memory. You don't see comedians go on stage with a giant teleprompter or a long sheet of a paper. That is the best part. They deliver it and they deliver like it is their mess. They own it. You must own your speech. The best way is too memorize it. By memorizing, you don't have pause during the speech to find your place or scroll through the document to see if you missed anything. Own it. Prepare. Memorize.
6) There are two - If you are giving a speech as a Best Man or Maid of Honor, remember there are two people that should be included in your speech and it is not you and Groom/Bride. Both of the Groom and Bride should be mentioned in the speech. I don't care if the first time was over spaghetti two nights ago. Somehow work into the speech that the Groom slurped up his noodle and got sauce all over his face and the Bride promptly threw sauce on her face so he wasn't embarrassed and that is how you knew that they were meant to be. Doesn't matter if the first time that you met the other person was on the wedding day. Mention both. There are two. Remember that.
7) It is a conversation - The misconception about speeches is that you are broadcasting it to everyone. Technically you are speaking to everyone, but you have to imagine that you are talking to one person. You are having a conversation with that one person. The whole thing is that most everyone knows what you are talking about and if they don't, they will be genuinely interested because it feels like you are talking directly to them. Make that eye contact like you would in a normal conversation. Throw in a "You know what I mean?" or "Ain't that true?". It will relax you and relax the audience.
8) Slooooooooooow down - This isn't the Kentucky Derby. Speak clearly and normally. When people are nervous, they tend to speed up their speech. Slow down. Pause after sentences so that they sink in. Speak at a normal rate. It helps to feel that confidence. If you are nervous, throw in a good joke to break the ice. Embarrass yourself to get the audience laughing. All this will help you relax and slow your speech down.
No one can tell you how to write a good speech. We all write, talk, and convey things differently. I can only help you deliver it, structure it. The speech must have a point, a reason, an example, and a point. Once you got that, you got the makings of a good speech.