I'm strictly using postcards now but should probably start using some online methods at some point. 

Business cards, flyers, postcards, bandit signs, any direct-mail campaign, these are some of the small expenses necessary to keep any real estate investor's business actually work.

I don't have a magic postcard or letter.

Postcard with a family photo is working great for me. 

For all of you who try direct marketing through yellow letters/postcards what's the success rate?

I used to download address's from ReboGateway and send out yellow letters and post-cards.

Example - send postcards to everyone in the neighborhood /subdivision about your property. 

They have anything from postcards (recommended as first contact due to lower costs involved), letters, zip letters (they have your business card attached to the side that the owner can tear off and save for later) and other great products.The great thing about this service is that you just import your mailing list (which you created through listability.com or some other service) then pick the piece you want.

. - I wouldn't stall the process of getting started just because you don't have a website yet).And yes, I've got some examples of postcards I can send you, and I can point you in the right direction on the offer-making process too. 

I send these folks a post card through click2mail or a hand written letter introducing myself and asking if they would be interested in selling their house to me.