A can of beer to chill her out - an hour before the vet is due
or a shot of stresnil once they get there (beer's better though IMHO)
Try if you can, to construct a very small pen or crush using gates/hurdles (don't give her room to move or turn round in it and try to have the side open so the vet can get in there to take the sample rather than going in from the top) and if you have a wooden brush shank, she'll bite on this and you can use it to keep her head still (in the absence of having a snare for her mouth/snout)
Try to stay relaxed yourself too (easier said than done, I know
) but she'll pick up if you're stressed - so deep breaths to soothe both of you and just keep talking to her to keep her calm.
Good luck
Karen x