Congratulations! You're a good enough friend of the bride and groom that you've been invited to make a toast.
Before you start building the toast, here are a few things you might want to remember.

1. It should be short.
   No, really short.
   
2. It should be complimentary.
   No "funny" horror stories.
   
3. It should thank them for the honor of representing their community and being their friend.
   
4. It should be about the bride and groom and not about you.
   Talk about why they're good candidates for this wonderful relationship and right for each other.
   
5. It always ends with a toast to their future.
   Wish them the best life possible.
   

I don't think I've really heard any speeches that don't do better for a whole bunch of preparation and a fair amount of practice.
That means that the wedding speech should really be written before the morning of the wedding so that you have time for both the preparation and the practice.
It probably means you should do it before you get on the airplane.
(Nobody wants to sit by you if you're mumbling to yourself and besides sometimes the movie is good on the plane.
Not a great option.
) Giving a speech is a responsibility you accept when you accept the honor of being best man or best woman, or if your child is being married.
So, do your homework, write something that reflects how wonderful your friend is, and prepare to shine.
Preparation first: I don't care how great a public speaker you are, it's important to be prepared.
You don't want to fall flat on your face here.

• Figure out the arc of the speech: thank you, how great your friend is, how perfect a couple they are, why marriage matters, and a toast to happy ever after.
  
• Search for that story that you can tell, and tell in a pretty short amount of time, that will really represent how wonderful your friend is.
  What are the most important points?
• Talk about what you've noticed in your friend's partner that makes this a wonderful match.
  
• Reflect on what you think marriage will bring to your friend, to this couple.
  Say how happy you are for them.
  
• What do you want them to have in their life together.
  It doesn't need to be the same old thing...
  and the same old thing is also lovely!
• Write them down in talking points.
  If you need some sentences, that's ok.
  But get (at least!) the outline on paper.
  
• Don't use quotes just to use them.
  Use them if they're right and they're important to you.
  
• Be as authentic as you can possibly be.
  Sound like yourself.
  Let your love shine through.
  

Practice.
If you never knew why bathroom mirrors were invented, this is it.
Take your notes, let your date or spouse know you'll be in the studio for a while, walk into the bathroom and close the door.

• Say it out loud.
  
• Practice the pieces.
  Make sure you have the segues.
  
• Rewrite what doesn't work.
  
• Learn it well enough that you only need to glance at the page (unless you're really nervous).
  
• If you're the nervous type, be truthful and disarming about it.
  People love it if you tell them what's going on for you.
  
• Practice letting yourself be honest enough that your emotions show through.
  

Then day of, tuck your notes into your tux pocket or evening bag, fluff up, take a deep breath and knock 'em dead with your wit and beauty.
Start your friends off right!