Being asked to give a speech at a wedding reception is generally considered to be an honor. After all, the bride and groom wouldn't have asked you to give a speech honoring them at their reception if they didn't consider you a dear friend (and someone likely to say good things about them!). In order to live up to this flattering gesture of trust, you need to make sure your speech goes off without a hitch and is the best that it can be.
Write a rough outline ahead of time. Think of all of the wonderful things you could say about the bride and groom and put all of these points down on paper. Search your memory for the earliest recollections you have of the two of them, and include some more recent points, as well.
Flesh out your outline into a full speech, then mercilessly edit it to keep it within 3 to 5 minutes in length. When editing, make sure you retain the most sentimental parts of the speech, as sentimentality is what a wedding reception speech is all about.
Throw in a few humorous anecdotes. The speech will be more interesting to your audience if they have a few opportunities to laugh. Three to four points of humor are usually adequate in a wedding reception speech.
Pause from time to time. This will give your audience time to absorb what you're saying and reflect on pertinent points. It will also help to keep you from sounding like a robot that's talking too fast.
Rest your hands on a lectern or keep them at your sides while you speak. Waving your hands about during a speech is distracting for the audience.
Look your audience members in the eye as you speak. A good rule of thumb is to move your eye contact from person to person in the audience, never retaining any one person's gaze for longer than 3 seconds at a time.
Keep a notecard of the bullet points of your speech in one hand and refer to it, if necessary. If you need to look at the card, only glance at it long enough to remember what you want to talk about, then go back to looking at your audience.