On the nitty-gritty side of things: what do you consider "a lot?" It's hard to say whether you're even sending enough letters. Also, how long have you been at it? Listen to every podcast that deals with wholesaling and you'll hear a theme - it usually takes many months, perhaps a year, and multiple mailings to the same addresses to snare that first deal. Wholesaling isn't for the faint of heart.
Now on to the most important part: WHY are you doing this? Keep that in front of you at all times. There's often a rush of adrenaline and euphoria at the start of a business venture, a rush that quickly flames out. Then what? Then you're into the grind, the daily reality, the realization that you're going to have more things go wrong than go right. If your reason why isn't strong, you'll give up.
A huge way to combat discouragement is to listen to the podcasts, read the blogs, and interact here on the forums. You made a good move by asking a question and seeking some help rather than flailing along alone. Network and meet other investors in person as well. Keep mailing. Keep moving. There's no one step that's going to be The Step. It's going to be thousands of small steps, accumulated day by day until they finally add up to something big.
