Usually it happens when the ROM download wasn't completed properly.. or when you flashed it it may have had an Error somewhere.

This can be corrected by removing your battery and waiting for about 10 seconds.. Replace it.
Hold Volume Down + Power until the ROM boots into Bootloader. Move to Recovery and try flashing the ROM Again. Don't forget to Wipe Cache, Delvik Cache and a Factory Reset prior to flashing the ROM.

If you get another Bootloop, you may need to try downloading the ROM again, or trying a different ROM.

You can also download Home of the MD5summer to check and Make sure the ROM MD5Sum listed on the website, matches the MD5Sum of the ROM once you finished downloading it. This helps make sure the ROM downloaded Completely and exactly.

 

Are you wiping data, cache and dalvik cache (advanced menu in Clockwork) before loading the Rom? If you're using Rom Manager, you might try wiping everything back to back directly in Clockwork instead, then load the Rom from SD card.