Anything at all - you'll probably be given a hospital gown to change into, and be naked underneath from the waist up. At least, thats what I always do for my mammos and ultrasounds 
Mammos are slightly uncomfortable, but very quick - and the ladies who do them have always, IME, been lovely. THey basically squish your boob twice, and take photos while you hold your breath (so as not to move). If its uncomfortable, I find counting backwards from ten helps - its always finished by the time I get to 2 or so!
They ask you to wait so they can check the x-rays are clear, and if not (eg if they didn't position you properly) they ask to re-do one. But thats fairly unusual.
Ultrasounds are totally painless, they just smear you with jelly stuff and slide the u/s wand over your breast, and up into your armpit, whilst the radiologist watches the screen. If its a doctor doing it, they will probably reassure you straight away - if its a radiographer, they may have to pass assessment on to a doctor to tell you, so don't worry if they say nothing.
Good luck FAB, seems a long time ago you were given the apt - imagine it will be good to get them over and done with.