One of the more alluring aspects of being a full-time musician is that no one is telling you what to where, where to be, or what to do. Essentially, your time is your own until show time. Very rarely does your professional demeanor begin when you flip the sound system on; it begins with a couple of steps that make a huge difference and impression on your client.
Show up early
If a client hires you for 2 hours, that does not mean that you show up 10 minutes before your set starts. You have to factor in traffic that may accrue as you head to the venue as well as simply finding the building. A lot of the places you are hired to play you have never been before so knowing exactly where you are going is time you should factor in. You also need to factor in time to set up all your equipment properly and figuring out where all of your electrical outlets are ( I always bring an extra extension cord just in case the outlet is far away). If you are playing in multiple locations (ceremony and cocktail hour) set up a duplicate sound system at each location or just set up the bare bones ( mic stand, speaker stand, etc). Coordinate with the DJ beforehand as well because they may have speakers you can use. They are used to providing sound for the whole day so they will most likely have spare parts. Lastly, but more importantly, leave time to walk the grounds, admire their beauty and tranquility, and talk to the staff so they know who you are. Wedding coordinators, property managers, waitstaff, etc. deal with musicians and entertainment all the time and if you stand out due to a bit of light conversation, that will do wonders for you. Bring business cards, mingle with the staff, and network prior to the performance.
Play to the room
You have a set list, go to songs that you love to play, but they may not all be the best songs for the environment. It may not be a good idea to play "All about that bass" at a kids birthday party whereas it may not be a good idea to play the song "Jumper" at a funeral. Look out at your audience, read the demographic, and then alter your set list in your head based on what you see. You will notice if you are doing well because people tend to turn to you and smile if you play songs they like, are familiar with, and can sing along to.
After your show is done, say thank you the wedding coordinator, property manager, the couple that got married, and anyone else responsible for you being there that day. You can say these thank you's in person, via a phone call or in a well written e-mail. It is important not just for your professional reputation but also for your well-being. As musicians, we must never forget how lucky we are to make people happy with music that we love to play. It is important to share this gratitude with others who not only listened but compensated us for our service and talent.