tight bolts

Does anyone have a good suggestion on how I can get a bolt loose.  It's not
rusted or anything, in fact I had it out just a few months ago, but the
boyfriend always "tightens them up a bit for me" but he's about 800 times
stronger, and I can never get them loose again.

And no smartass "don't let him do it anymore" comments!  That's already on the
way!

TIA (I think my transmission is leaking oil again, but I can't check it
without removing the bolt!)
Patricia
'59 Bugeye