If you are doing direct mail, I recommend a cover letter to introduce yourself and your business with an overview of your experience, areas covered, etc. IF you have previous experience it's helpful to add to the letter that you have a resume upon request.
Post cards vs. Brochures I usually only recommend postcards for either special offers, deals or a second mailing. Your first contact should give the potential client ENOUGH information to call you, really hard to fit enough info needed on a postcard. Save those for when you have a special deal or if you are doing a second or followup mailing. A better fit would be a brochure.
Some may disagree with me on this one, but I know for me when I get a direct mailing from a company I always scan for contact information and a website address I can visit to get more information, and I want that information to be different yet appealing. To me that says a business cares and has made the effort to cover all angles. To me it just screams "I'm brand new and don't know if I'll succeed so I didn't build a website". That's just my opinion.
Another tip is to try and obtain preliminary information for your mailings. Yes this can be time consuming but you can break it into two segments. If you are doing a 1000 piece mailing you can split that into groups of 4 which will also help you track and manage contacts so you can see which are getting looked at. Take one batch and include addressing an office manager by name or put Attn: Billing Office Manager. This makes the critical statement that you are NOT out to grab someone's job and that you are here to offer help/assistance to the billing and office manager, NOT get around them. Do not try and get around the office manager, you will fail every time. Also do not address personal and confidential to the provider, again this shows your being sneaky and trying to get around the office manager. The office manager is the gatekeeper, they are the most trusted in the office and usually what they say goes, and the doctor almost always trust their opinions and requests. That is the golden rule I live by when marketing DO NOT TRY AND GET AROUND THE OFFICE MANAGER, YOU need her anyway!!
If you are doing brochures, another tip I have is NEVER to blend photo's with clipart!! It looks very unprofessional. Try to get professionally printed materials and photo's are more professional than clipart. NEVER forget the business card. Try to avoid magnetic cards as those can be dangerous if opened around a flash drive or computer or phone (yes I've seen it happen where magnetic card demagnetized a phone once)
This is probably more than you asked for but hope it helps. It's a good idea to make sure you have a good contact management software to keep track of all your mailings so that you have good information at your fingertips when a prospect calls you and it also gives you a detailed view of each office. For example, I use Act. I enter all my prospects into a database called Prospects, if I do a mailing, I enter in the type of mailing and any details, such as did I send a colored envelope, did I hand write the envelope, did I address the office manager by name? Then if I get a call from that office I pull them up and I have all the information I need to know that I had a good mailing that got their attention. If I pre-screen my mailings with a phone call previously I note every detail of the call. Perhaps I talked with the office manager for 10 min and she mentioned that she has a kid in college, and I ask if I can send her some information, she says yes.. when she calls, I can ask how her kid in college is doing and she takes note that I remembered something like that. Maybe I spoke with a cranky receptionist, that information can be just as important. Making a connection is important since the rate of return on direct mail is often 1% or lower. The more exposure you have to potential clients the better, again another reason to have a website because it's something for them to also remember you by.