There is no enforced or officially recommended naming convention, other than the usual rules for identifiers and what your compiler tells you. Although a number of known conflicts has already some kind of workaround applied, there are still plenty of possibilities for conflicts. You have to find out what works for you.
The single best option is to just rename the conflicting field in the IDL file, or maybe add an underscore. Regarding RPC and serialization, the field name does not matter, only the field ID is important. Thus renaming a field is not a compatibility breaker vis-á-vis the serialized data, only regarding the source code, which is a fixable thing.
EDIT: Here is a nice example for an naming issue that is still open. Due to the nature of the problem, it manifests only with one very specific language: Go. You will never run into troubles with that IDL as long as you stick to any other of the languages supported by Thrift. Nevertheless, things like this should be fixed.