I agree with Ian,
Make sure you have a plan and have discussed it with the couple before hand. Be aware of the time available between the end of the service and the wedding breakfast (and the distance between the two) - this will give an indication of the time for you to do your thing. Work fast, but effectively - if it's cold or wet, people will not want to wait around for long. They may be friends, but they will still be expecting you to deliver the goods.
Create a list of what photo's you intend to take, of whom and in what order, what emotions do the B&G want as a theme - happy, romantic, humourous, formal etc. (you can include them all, but it's nice to have a central theme). and make sure the B&G agree before the day. What is the final presentation media - album / photobook / mounted prints etc. who is paying for it?... How many people will be present (family / friends / onlookers - helps estimate the number of photographs you can take in the time available). Also, any additional prints that family and friends want is an opportunity to make some money to pay back for the flash you're investing in. Try and agree a specific number for the 'album' this will give you a definite target of 'keepers' to focus on.
Do the group shot's first - they can then move to the bar while you concentrate on the Bride & Groom, Bridesmaids, Ushers etc. Don't make way for snappers - yours are the ones that count and you need to use the time available.
Make sure you set the white balance for indoor shots, and back again for outdoor shots..
Get some candid shots as well, but only once the 'main' shots are completed and in the bag! Don't constantly check your images on the screen, make sure your happy with the view, DOF and exposure etc.
before you click - other wise you are wasting valuable time. Have at least one spare battery, fully charged, and extra's for the flash. If you have planned your shot's in advance then you shouldn't need to keep swapping lenses between shots which is also a time waster and distraction for the subjects. Beware of bright sunny days - they are a killer for exposure errors at weddings - that large white dress can reflect a lot of light and it's easy for the camera to be fooled - coupled with the hard shadow very bright light brings it can look awful. Try and work in open shade if possible. (if in doubt - expose for the hi-lights and tone them back in photoshop later, but beware of noise). Also, don't be afraid to up your ISO if the exposure is too slow - it's easier to correct noise than camera shake.
Remember your viewfinder generally only shows 97% of the picture area - make sure there are no 'unwanted' bits either side or above below the image or you may need to spend time cropping later.
Checkout the venue - look for locations that will cover most weather options (bright sunlight, heavy rain, big enough for group shot's small enough for intimacy etc.) If you are forced indoors, make sure there is an area set aside that is a pleasent backdrop to the photo's and watch out for light from windows - if it's there find a way to use it, otherwise it will ruin your shots). If it is a church, watch out for the gravestones - in my opinion they don't improve wedding photo's
Take a tripod. If indoors - look for stairs, they can often even the distribution of people during group shots.
Research - see if any local photographers have worked the venue previously - review what they have done and how you could improve it. If possible, identify some set pictures (trawl the internet, you will find plenty of examples) and try and recreate them using friends to get familiar with working with the people and obtaining the right poses. Again, think of how you might improve the shot, or creatively change it. Make sure you have practiced with your new flash and are comfortable with it before the big day. Try not to use direct flash (pointing straight from the camera to the subject - try and bounce it, or for a few pounds get a plastic diffuser) This is the most difficult to get right, so practice, practice, practice.
Don't forget to photograph the accessories - Cake, rings, flowers, car, etc. Watch out for the finer details (background, what is in/out of focus, where the shadows fall etc.)
Stay calm - be assertive, use the Best Man and Chief Bridesmaid to organise people -Give them the list of photo's so they can have people ready. If possible take a friend who can assist (carry or protect equipment - hold the brolly that you have remembered to take with you, swap batteies in the flash etc.)
Keep talking - encouragingly, the subjects will be as nervous in front of the camera as you may be behind it - make sure you are ok with the way they are standing, and that they are relaxed and happy before you look through the viewfinder - forced smiles never look good, pay attention to detail - the bit you miss will be the most prominent eye catcher on the print for sure. Use a technique for making sure all eyes are looking at you and open!
Leave a card at the reception / with chauffer / church etc. - they may be able to promote you for other business if it goes OK and you enjoy it.
Have fun - it is a wedding after all!
Good Luck and post the best shot back here so we can see how well you did.
Phil