By far the simplest option is just use the ARC conversion tool under Edit > Refactor.
It's not very clear, but basically you just check any files you want to use ARC and uncheck any that you don't. This will do the following:
1) Files that are unchecked will have the -fno-objc-arc flag applied
2) Files that are checked will be converted to ARC
3) Files that are checked that have already been converted to ARC will be untouched
Read that again carefully. What sometimes confuses people is that they assume that files that are already converted should be left unchecked when running the tool again, but this will incorrectly apply the -fno-objc-arc to those files (resulting in leaks).
You should think of the ARC Conversion tool as a "batch-select ARC compatible files and convert files that aren't already ARC" tool instead of a straight conversion tool.
You can run the conversion tool as many times as you want - the effects are not cumulative (although converting files to ARC is not reversible).