Plan as much as possible
There's no such thing as too much preparation. Start by sectioning your thoughts into areas you want to cover first, it'll make it easier to focus.
Consider your audience
Are you absolutely sure that joke is wedding appropriate? Think about who will be listening and if in doubt, run it by someone else to see what they think.
Make it personal
Practice reading it out loud after you've written it. Does it come naturally to you? Does it feel authentic and from the heart?
Keep an eye on the time
Wedding speeches traditionally vary in length depending on the role, but a wedding speech should be between 5-10 minutes long.
This is the most important point. Remember why you're giving a speech - it's either your wedding day, or the couple have asked you to speak - so enjoy it!
Example Wedding Speeches