Don't "wing it" with this one. You don't have to write out your best man toast verbatim, but you should mark down some clear notes and cues that will direct you towards what you want to mention.
Keep it short and sweet.
That being said, brevity is key with a wedding toast. And although toasts are such a traditional aspect to a wedding reception, they also can get tedious. You really don't need to speak for longer than a minute or two. Tell an anecdote or memory of the groom or couple and wish them well.
Don't tell that story.
It probably goes without saying, but you really shouldn't tell an embarrassing story about the groom or "teasingly" insult the bride. The goal is to offer them congratulations, not give them a Comedy Central roast. Instead, share a memory that is positive, charming, and uplifting to set the right tone.
It's most important to be yourself. As the best man, you probably know many guests at the wedding, and they'll see through a poseur act. If you are normally cracking jokes, it's fine to be humorous in your toast. If you are a English teacher, a Shakespeare quote might make perfect sense. But if you've never even seen Romeo and Juliet or can't deliver a one-liner to save your life, then don't include those elements. Just be you.
Settle pre-toast jitters.
If you aren't used to public speaking, giving a best man toast can be really intimidating. Totally normal. Calm yourself by taking a few deep breaths and sip some water if your mouth is dry. Although it might be tempting to have a couple of quick cocktails to settle your nerves, try to avoid that. A slurring best man toast is just not cute.
Be friendly and engaging.
Don't just stare at your index cards. Create an instant rapport with guests by making eye contact throughout the crowd and with the bride and groom, smiling, and speaking in a confident, clear voice.
Remember a conclusion sentence.
So, you've told a sweet story. The bridesmaids adore you. You're a hit. But then you abruptly sit down and everyone pauses before awkwardly clinking glasses. Don't forget that all-important conclusion sentence that signals the toast is over, such as, "So, congrats Jane and John," or "To the happy couple," as you raise your glass.