Every year I take at least one business trip. For the most part, when it comes to packing, it's not all that different than getting ready for a vacation. But, there are a couple subtle differences.
First thing to do is take care of the basics: clothes, shoes, bathroom sundries and so on. Pack these as you would for any trip. See my other posts about traveling. The only thing you might want to augment is the type of clothes you're packing. If your trip is to attend business meetings and courses, you'll want to pack the appropriate amount of professional wear. If you're traveling to do some hands-on work (in my case: organizing, sorting and unpacking), you might want to pack some more utilitarian outfits.
After the basics are taken care of, tackle the specifics. If you're attending classes, do you need to bring your own notepad and pen or tablet/laptop for note taking? Business cards? Resumes? If you're doing hands-on work, will you be able to bring whatever supplies you normally have at the ready at your place of work? For instance, my work bag includes rubber bands, markers, sticky notes and a measuring tape. I need to decide if I'm able to pack and bring these items or access them at my temporary location. Perhaps that warrants a phone call to the destination or even sending a package ahead of time.
Will I be staying at a hotel or with colleagues elsewhere? If a colleague has offered their home to me, I should ask what items I'm expected to bring (towels, for example) and what will be in the guest room.
Lastly, don't forget any documentation you might need for this business trip. Contracts, correspondences, informational sheets, examples for the clients to peruse. The last thing you want on a work trip is to forget that it's not vacation, but work after all!