If the bride and groom haven't requested an unplugged ceremony and you are wanting to take pictures, here are some things to consider.
Show up early. This will give you an opportunity to be a little more choosy about where you sit.
For the ceremony, consider sitting at the end of the rows, opposite of the aisle. This way you aren't obstructing other guests' view and distracting them when you lift up your camera to take pictures.
At the reception, if possible, pick a seat that is close to the dance floor. This is where most of my favorite photography moments happen.
Set your camera on manual. Unless it's a well-lit venue, a camera set on automatic is going to produce blurry pictures. If you don't have an option to set your camera on manual, brace yourself. Find a vacant chair or find a stationary object that is tall enough for your camera to see the bride and groom. You can use this object for stability. Try using your flash as seldom as possible. This can be distracting, especially during the ceremony.
For cellphones, try apps like “Camera Genius” or “Camera+” for image stabilization. This will keep your camera on your phone stable so that you can take pictures in low light without much motion blur.
Get close! I love when guests get close to the the dance floor to take pictures, or when guests are surrounding the cake while it is being cut. You'll never be able to capture the emotion from your seat unless you are seated right beside the area where these events are taking place. Try out the Olloclip for cellphones. This will give you the option to capture close up pictures (macro), as well as wide shots of the reception/ceremony venue.
For instagramming details: Bring your wedding favor near natural light (a window). If the sun is shinning through the window at the cake, take a picture of it with the window at a 90 degree angle. You can take professional looking pictures with your cell phone with the right light. I don't recommend that you do this after the reception starts. It's just something to do before the bride and groom arrive. Once the bride and groom arrive, it's time to celebrate with them.
Just remember to enjoy the wedding and be more in the moment rather than worrying about capturing the moment.