Sending up the groom at every opportunity
Start your speech by breaking the ice (SEE EXAMPLES), then announce an intention to roast rather than toast (SEE EXAMPLES). That way, everyone has been pre-warned on what's about to happen.
Once that's out of the way, start in earnest with a piece about the honour of being asked to be a best man and how, in this case, it doesn't apply (SEE EXAMPLES). Get in early with some remarks about the groom's appearance (SEE EXAMPLES), his anxiety (SEE EXAMPLES) and his nerves (SEE EXAMPLES) on the day. In specific cases, his performance during the service (SEE EXAMPLES) and/or his attitude towards the wedding organisation (SEE EXAMPLES) are ripe for send up too.
Then of course, there's the stag party. You could use this very effectively to add more colour to an already unflattering picture of the groom (SEE EXAMPLES).
By now, the gloves are well and truly off. Move onto the groom's personality quirks (SEE EXAMPLES), make light of his hobbies (SEE EXAMPLES) and career (SEE EXAMPLES), and maybe talk a little about his unremarkable past (SEE EXAMPLES). Remember, only make fun - don't outright humiliate! If you're not entirely sure which is which, take a look at this handy advice.
After all that, it's time to explain just what your role is (SEE EXAMPLES), before saying something funny about your association with the groom, such as how you met each other (SEE EXAMPLES).
You could also comment on your reaction to the news of the happy couple's engagement (SEE EXAMPLES), how surprised you were given how unromantic the groom is (SEE EXAMPLES) and how beautiful and lovely the bride is (SEE EXAMPLES). Share views on how she's improved the groom (SEE EXAMPLES) and how they are destined to have a great future together (SEE EXAMPLES).
Bring the speech to a close with a single acknowledgement that the groom is not all that bad really (SEE EXAMPLES), before finishing the speech with a couple of parting digs. Perhaps you could mention the evening disco and what a rubbish dancer the groom is (SEE EXAMPLES), or how he has failed to impress with his choice of honeymoon (SEE EXAMPLES) - it's completely up to you.
Last but most certainly not least, toast the newlyweds (SEE EXAMPLES).