Having done a few now, I'd say;
a) don't sweat it too much. It's a happy occasion and folk are in high spirits, and people will laugh at any old crap with sufficient free booze in 'em.
b) try to avoid too many clichés – a few are ok but everyone's heard stock lines before. Better to tailor it to the couple, presumably you know them well or you wouldn't be doing it. Last BMS I gave, I got a World of Warcraft joke in there, then followed up with "for the benefit of those who don't know what I'm talking about, that was really funny" – and got away with it.
If there's two of you, don't do a line each, that's just painful. Either defer to the more confident / better speaker to take the role, or do half and half (or 60:40 or whatever).