You'll have little to no in-put.
The producers will sit down with you and ask for some ideas and you must create a presentation as to what you can deliver. Being a clothing thing I'd suggest routines that involve clothing (sorry I don't recall the names) but the Quick change T-shirt effect might be appropriate, the appearing neck tie, color change neck tie, the color change vest, etc. are a few of the things that come to mind. You'll also have to think like Criss Angel in some cases, teaching an actor how to do certain of these color changes so you can do the magic to them; transforming what they wear to being something sold by your client. . . not camera tricks but certainly the use of a "trained assistant".
You MUST get secrecy agreements with everyone directly involved with the magic side of things. This is simply a legal protection for you that encourages people that you trust with "your" secrets to keep their mouth shut or else face legal action. While any contract is only as good as the lawyer behind it, it's been found that most people will honor said promises and too, it makes you look far more professional.
In the case of modified outfits (for creating the quick change effects) the company must cover all said costs and it must be worded in your contract that the modified clothing remains your property after the shoot. . . but it will put an obligation on your shoulders to feature said pieces in shows for the balance of the season, so if you are doing an early spring shoot you will have to feature the clothing used in that shoot for the 12 weeks spring is generally seen as existing. This includes featuring the company's logo on your posters and notation in other show related materials.
NO, this is not a common practice but is an encouraged practice for someone new to the corporate environment and seeking to grow in that world. Companies love the expression of loyalty and support; knowing that you're not a one show pony as it were. they fee they pay you is for more than the one gig and in a crappy economy such as we're in, the added (though subtle) acts of consumer contact will be more than appreciated.
AFTER THE FIRST SHOOT (even before) you will want to start looking for other ways of working with the client. Grand Illusion offers several advantages when it comes to clothing lines such as the Costume Trunk and my own Wordrobe Closet routine and a number of "Bordoire" type settings. I think the biggest advantage however, is to develop new effects or serious twists on the older one, that will allow you and the magic to stand out. BUT DON'T FORGET TO CHARGE THEM! There are development costs.
I'm more than glad to help you on the effects side of things but when it comes to the business end, this is pretty much the extent of things for me.
Best of luck!
- Jesus Puerta