Have a search of the forum, there's TONS of advice there which will probably include things you've not even considered.
I would say though, managing the expectations of the 'customers' (ok, they're friends but you know...) is important. Many people see someone has a DSLR and assume they're the worlds best photographer without understanding that actually, to shoot in a lot of situations (despite a good eye) you need a lot of some seriously expensive kit. Add to that nerves, inexperience etc and it soon can become a headache for you and then the bride is disappointed because your photos don't look and aren't presented like like they would if they'd hired an expensive pro.
That said, I played Tog no. 2 at a recent mates wedding with nothing more than a D90, kit lens and SB400 and the 'customers' were more than happy - it's just about managing their expectations, knowing the limitations of your kit and shooting within them.