If you previously had your phone rooted with unrevoked 3.21, just reflash your recovery then flash the superuser app found here
[APP] Superuser 2.3.6.1 - Now on the Market [2010-09-05] - xda-developers
Easy as pie, good to go.. No need to "RE-ROOT"...
Enjoy,
-CC
P.s. Original long root method, has links to the recoveries
[GUIDE] How to root Android 2.2 on the EVO 4G. If you want more complex, you could also follow this for the latest OTA already "rooted" for ya
[ROM] joeykrim-SDX-original-4.0 Odex *Stock Rooted 3.29.651.5* Sept 23rd 2010 - xda-developers