A large portion of my interaction with opposing counsel and potential clients is through email. Sometimes, however the ease of email makes lawyers forget basic rules of email etiquette. If most of your correspondence is done via email, be sure you have a proper signature block.
Include the basics
Your signature block should include your name, firm name, address, phone number, fax number, and email address. Your name should be whatever you put on court documents—this is not the proper email address to list your college nickname. Your firm name and address are fairly self-explanatory.
For your phone number, if you are listing your direct number it is helpful to explicitly say that. Listing your email address in the signature block might seem redundant, but I have received emails from other attorneys that do not come from the address listed.
You are now in the realm of personal discretion. If you have a blog that you update frequently, and it is related to your practice of law, you might want to list it at the bottom of the signature block. Personally, I only think you should list it blogs that are active. Even then, I think it is a judgment call. If you list it, people will go there, and you want it to look good. If you have not posted anything in two months, that is not good.
You can list a Facebook page, or a Twitter account, but I do not recommend it. One, do you really want potential clients and opposing counsel checking out your Facebook page? Two, your signature block is probably fairly thick at this point, so you really only want to list relevant stuff.
Keep it simple
Your signature block is up to you. I am not a fan of bolding your name, putting your firm name in blue, or adding a logo. That is just my humble opinion. If you feel inclined to do that, try to keep it simple.