...
If we are talking about timming spaces or doing smth else with the elements, it would be better to split string, do manipulations and concat it back.
or use well known way of removing duplicate spaces (http://www.sqlservercentral.com/articles/T-SQL/68378/) with one more check...
declare @a varchar(500)
set @a = ' Alexandria , VA , Arlington , TX '
set @a = REPLACE(''''+REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(LTRIM(RTRIM(@a)),' ',' '+CHAR(7)),CHAR(7)+' ',''),CHAR(7),''),' , ', ','),',',''',''')+'''',''','' ',''',''')
select @a