First off, let me say that a dress check is a marvelous idea. The chapter may hate it, but it's a necessity. May I also recommend a make-up check? At least that's a necessity in my chapter.
A comment about bump groups: They take A LOT of work if you want to get it right (I'd suggest starting to practice them now to use them last year; . Our chapter tried them last year, and the rushees said they felt like they were being passed off from one member to another and they didn't like it at all (quota was 18; we got 8, if that gives you an idea). Aside from last year, we've done rotation groups rather than bump groups, and they've worked MUCH better. The rushees just feel like they're meeting a lot of girls, and they tend not to feel like they're being abandoned every few minutes. With either technique, though, try to make sure that no Phi Mu has the same rushee for more than one night (excluding Pref Night, when groups aren't usually used); they can catch on pretty easily to the fact that groups are being used, especially if you're using bump groups.
Okay, now as for planning the workshop: Try to schedule it in a room in your student union or some other 'neutral territory' if you will (you know, where the girls can't escape to their room), with a lunch break in the middle (but one where you all have lunch together). Make it a rule that everyone needs to turn their cell phones off, they need to come dressed as if it was already Recruitment time, and they need to keep a permasmile as if they are recruiting someone at that very moment. Try to get in the habit of being 'stuck' together for an extended period of time and of impressing new people every second. The more you do that, the more natural it will seem to them -- and the rushees -- come Recruitment time. You may want to end with a sort of gavel pass (or something to that effect, whether you physically pass something around or not) where each of your members says what she's found most useful from the day and what she still feels she and/or the chapter needs to work on.
Best of luck to you and your chapter with the workshop and Recruitment! I'm sure you have it in you and you'll do great during Recruitment. Let us all know how it goes!