I'm going to be Trisha Elric again this year! I can't say I have any tips, though. I would think it would be troublesome to make or expensive to buy the military outfit. Make sure to have the accessories (eye patch or glasses) as close as you can get them. The hair doesn't have to be perfect to be recognizable.
Out of curiosity, which FMA? Just the regular anime? (Where King Bradley is... he was Pride, right? Yeah. Not the one that follows the manga where he's Wrath.)