1. A good start is to jot down bullet points of what you want to cover. Take a week for this, and keep a notebook on your nighttable.
2. Find a good flow between the bullet points, you need some kind of structure so people can follow (ex for a wedding speech : start with your friendship, then talk about the relation between your friend & his wife, and finish off with some wishes to the couple)
3. Andecdotes : the audience does not want to feel excluded : make sure anecdotes can be understood by everyone in the room
4. Find some quotes that summarise or colour your text. Google some words/characteristics that you want to illustrate and you'll find plenty of intelligent well-written lines.
5. Use CTRL F7 in word to find synonymes if you repeat the same words too much
6. Read it aloud a few times : it needs to flow in your mouth (Use exercices of style to make it flow better, like alliterations (words starting with the same letter) or use of adjectives per groups of 3...
7. Make it short and crisp : Maximum 7 minutes