There are two ways of thinking when it comes to how a designers branding looks.
1. Why should a customer trust me to design for them if my stuff doesn't look polished? How can a customer trust me to design amazing stuff for them when I don't even do it for myself? How am I going to give advice on branding and convince someone to invest in quality design, when I don't invest/put the time into it myself?
2. I design non-stop for other businesses, events, causes, sports teams, etc. and trying to make them look good that I put very very very very little thought or time into my own brand. As far as I'm concerned my own branding should look terrible or sooooo simple that when someone asks why my work isn't polished, I say "it's because I focus all of my time into my client's projects and making them successful that I put zero time into my own brand."
The truth is probably somewhere in the middle. I don't get much time to think about the way my stuff looks. Also, for some reason it's difficult to make designing for myself a priority, and in talking to other designers, many of them have the same problem. Ideally though, I have thought and creativity in everything I show or do for my own branding.
What could be more boring than an invoice? No-one wants to get them, pay them or look at them (maybe accountants do I guess, who knows). Anyway, my invoices looked probably pretty similar to everyone else'. Simple, professional, gets to the point and shows the numbers. There is nothing wrong with that and I haven't had complaints, but recently I wanted to updated them.
I made a template that looks like a Pantone color swatch. I figure the info is still easy to navigate, but I can update the color and Pantone swatch name every couple months or something. Anyway, I have only sent a couple out so far, but hope it makes a client smile at some point. Even if they don't exactly love paying an invoice :).