I have played at numerous wedding ceremonies over the years, and it is something that I really enjoy. I have done a lot more of it over the past two years, since quitting my job as a lawyer and going full time with music/Yoga Rhapsody.
I have a standard format which I find works well, but of course this is entirely customisable to the bride and groom. What I generally do is sing three songs at the ceremony - one when the bride arrives and walks down the aisle, one during the signing of the registry, and one at the conclusion of the ceremony, as the couple make their exit. I take requests for these songs, and consult with the couple to find songs which are special and significant to them. Leading up to the ceremony, I send the couple recordings of my versions of these songs, so that they can make a final decision on their ideal musical accompaniment.
I have a PA system which I set up for the ceremony, which can then be used by the celebrant and any other speakers. I have a wireless headset microphone that the celebrant can also use.
Following the ceremony, I will generally move my PA system to the reception venue, set it up and play a 1-2 hour set during pre-dinner drinks/canapes, to kick the festivities off. I have a large and varied set of covers, and am also happy to consult with the couple as to songs that they would like to be included in this set.