One of the keys to professionalism as a freelancer is the ability to adhere to a style guide and produce work not only of a consistent quality, but in a consistent way, so that it looks like it belongs among the client's other content.

I know this is an area where I can improve, mostly due to being easily distracted by small shiny objects and forgetting what the style guide says for a particular client. Ways I deal with this include keeping their style guide reference open so I can easily consult it, or creating styles in in, say, my word processor so I can just apply the style "Clientname: Subheader 1" and move on. Some clients even send templates so you can just apply their styles--though these templates have the complication of needing to work with your software.

If you aren't given any style information, or you're working on something yourself, writing up a style guide as you go isn't a bad idea. For just a short, one-off project that's probably overkill, but say for a blog it can be quite useful. Otherwise you're in the middle of doing something and have to stop and wonder whether you only capitalized the first word in your titles or not. And yes, I speak from experience.

Other aspects of style you'll want to keep consistent are the voice, look, and tone (formal or casual? industry-speak or for laymen? refer to yourself or be the disembodied narrator? what colors? what fonts? what intro music? what background?)

Think of a consistent style as the polish that can really make work look professional. Inconsistency can look sloppy and amateurish, detracting from what otherwise might be brilliant content. Now pardon me as I go back through my blog and make sure I've taken my own advice.