What are some tips (structure & design) for dealing with tiny, older bathrooms? (best bang for the buck remodel ideas, better use of space, etc)

I'd use a sledgehammer.

The space between the studs on INTERIOR walls is wasted potential storage space. On EXTERIOR walls, that space is needed for insulation. On interior walls, you can knock holes in the drywall and insert shallow cabinets or shelves for anything from toilet paper to magazines.

A pedestal sink works wonders for making more room in a bathroom. Or a narrow stand with a bowl sink works too if you need the storage space.

I also love having a niche built into the shower/tub wall. It can bump out into the wall space instead of taking up room in the shower area. Bottles, soap, and other things can stay out of the way safely and drier.

Don't forget about being able to use the back of the door for towels or hooks. I've seen some people use a shoe storage holder on the back of a door for spare towels. It looked really nice and added nice color.

Build ’up’ as much as you can.

Use a pedestal sink if you can, to eliminate the under-the-sink cabinet. Helps open it up a bit.

Use mirrors to help ad the illusion of depth.

And as some one else stated, using lighter colors. Darker colors leave more pronounced shadows, giving it a smaller, even creepy feeling.

And when you’re done… don’t use one of those little rugs that hug your commode. Use a small throw rug you can slide close on those cold winter mornings.

You want to make your bathroom look bigger.
Use light colors and modern fixtures that fit into your budget
Hint: Look at local markets or maybe yard sales for fixtures and accesories