EVP are like any other recording. The better the equipment, the better the results. What are you using to record? Tiny digital recorders tend to create a lot of noise. I recorded my best EVP using a professional mic, a lap top, and a pre-amp, filter. Some digital mics, like condenser mics, have a filter built in. The recorder must lay on a flat surface, undisturbed, for optimal results.
Also, you have to run the recording through software like Cool Edit Pro (my all time favorite) to increase volume, filter out hissing if necessary, and to analyze the sound patterns. For example, you can recognize possible speech because there is a wave instead of the a straight or speckled line. If you can't hear it, you can use the software to increase the volume of that piece. Hope this helps.