Two things you can prepare for in advance, firstly have a wet weather plan, scout an alternative location should the weather turn, organise yourself and equipment for a change in light conditions. Monitor the weather the week before the wedding to prepare yourself should the worst case situation happen. Have some attractive indoor locations in mind as well, should the sky darken and the heavens open. Always keep an umbrella close; you never know when it might come in use. They are great with rain but also useful with very sunny and overcast weather as well.
You can also keep an eye on the weather, is it a storm that's settled in for the day or are you going to have clear skies in an hour or so. Remember that dramatic sky can make for exciting shots as well and the colours always seem to be more alive after a storm. So long are you prepare you can turn the situation to your advantage.
Another way around bad weather at wedding shoots is to extend the photography time and take the shots between showers. Line up the camera while it's pouring down and enlist the help of a willing guest, the best man or an usher. Have them to hand so that you can round up the appropriate guests between downpours. By organising your shots you can rapidly with a little help get the wedding album complete without to much fuss.
Keeping on top of the situation will calm the bride and grooms nerves too, it will create a professional image on your part and by being prepared you will still get those wonderful memories for the couple.