Conditional ALTER TABLE

Is there a way to make a conditional ALTER TABLE command?

If I want to drop a column only if that table's column exists, what  
would be the correct syntax.  I have tried several IF EXISTS, but I  
get an error at "IF".  I would like to use the Query tool in pgAdmin  
III, if possible.  But, if push comes to shove, I'll resort to psql.

As an alternative, how best to tell SQL (Query tool) to continue  
processing a script even if an "error" (like no column exists) is  
encountered.

Thanks,

Dale