something fun we did this past year that i set up was to have exec (or whomever) make the alphas (new members) sundaes just how they want them - they 'take their order' and make the sundaes in those little clear plastic sundae dishes, and on the bottom (on the outside) you write the name of their big sister (backwards, so when you look down in you can read it) and then put the sundae on top -- as long as the girls have some idea that 'after we have our sundaes we'll be having diamond (big) sister revealing, they will catch on quickly and usually jump up and run to their big -- who has some type of (uniform) present.
everyone seemed to like it, it was really successful last year. also an idea i heard at districts that we might use sometime is if bigs are being revealed at retreat have the big leave them pajamas (tasteful fun ones) as a present before retreat, then when everyone changes before bed the bigs have matching pajamas, and you can do a candle pass after the excitement dies down before heading off to bed. good luck with big sis revealing in your chapter!