Things that I found while getting my radios to work:
If you read and, closely, follow Robin2's simple rf24 tutorial you should be able to get them working. That tutorial sure helped me. Run the CheckConnection.ino (look in reply #30) to verify the physical wiring between the radio module and its processor (Arduino).
Make sure the rf24 power supply can provide enough current. This is especially true for the high power (external antenna) modules. I use homemade adapters like these. They are powered by 5V and have a 3.3V regulator on the board. Robin2 also has suggested trying with a 2 AA cell battery pack.
If using the high powered radios make sure to separate them by a few meters. They may not work too close together. Try the lower power settings.
Reset the radios by cycling power to them after uploading new code. I have found that to help. They do not reset with the Arduino.
Switch to 1MB data rate to catch the not so cloned clones.
'radio.setDataRate( RF24_1MBPS );'