I do enjoy a good best man's speech, but agree with the above, try and keep it under 10 mins. The punchier the better and people will view you more positively if you don't start to bore them. Better to leave them wanting more than have them looking at their watch.
Loads of ways to make it funny without it being a full on "roast", couple of one liners to get people warmed up. I agree be wary of ones done to death though, avoid websites with best man jokes etc. A good approach if you have other mates or close family in the room is to make a joke at their expense instead. Something along the lines of "Dave told me they'd organised the seating so those who'd spent the most on gifts are nearest the front (pause) just checking, can you hear me ok at the back there Bob?"
Then a funny anecdote or two about the groom, make the necessary than yous/toasts and you're home and dry.
People aren't expecting a stand up comic, they'll hopefully all be in a good mood, be slightly inebriated and want to laugh. Plus laughter is infectious so if you can land a couple early doors it's surprisingly easy to keep the momentum going.
Delivery is probably more important than content anyway. You don't need the comic timing of Peter Kay but be confident, head up and make sure everyone can hear you clearly. Try and enjoy it if you can, people can sense it and will laugh more, even if you **** up your lines or fumble your words a bit.