I do the same thing with the toothbrushes. I got a 5 pack of toothbrushes with travel caps on them for a $1 in Dollar Tree. I throw them away the last day of the cruise.
I also put my items that may leak in ziploc bags--body spray, hair spray, etc., before I pack them so they don't mess up my clothing.
If you need more than those small travel sized shampoos, plan ahead. Stop using your "opened" shampoo when it is half full, and put it aside to pack for your trip. OR if you can do without your regular for a few days buy a 2 in 1 (like Pert Plus) so you don't have to pack shampoo and conditioner. (Some ships also supply shampoo) This will make your luggage a little lighter anyway. I do the same thing with my lotion because I use a specific kind, and use it a lot. (I have reallllly dry skin)
Pack a few extra hangers if you like to hang everything up. I have found that there aren't usually enough hangers for what I bring PLUS what my roommate(s) bring(s).
Sovereign of the Seas, Royal Caribbean 11-17-03
Fascination, CCL 2-14-05
Fantasy, CCL 2-05-06
Elation, CCL 9-24-06
Carnival Victory, CCL 1-28-07
Sovereign of the Seas, Royal Caribbean, 9-24-07