For me, I need to have my photos ready first...meaning choose the subject, date, occaision of your photos, knowing what photos you hope to "get through" will help you plan what to bring. I always hope to get tons done, and end up lucky to get 5-8 pages. I work slowly and meticulously, and try to get as many photos on a page as I can. So realistically, I can get 2-4 events scrapped.
Next I broadly choose papers to coordinate with my photos, not going as far as creating kits, but also not including summer papers if I am scrapping thanksgiving or christmas. If I plan to use letter stickers, I include those that match the color or style of papers, same with embellies etc.
As far as tools, I bring the usual paper trimmer, scissors, adhesives, craft mat, a couple pretty punches, circle cutters, and markers,,,whatever will fit in a tote. I try to take no more than a milkcrate of papers...which is still too much, but oh so fun!!