First floor windows

I used to see tin foil used, but not so common anymore (and in some surprising places, too - I think 30 5th Ave had one).

People use the shades which draw from top to bottom (so bottom half of window has the shade, but top half can stay open for light). It works exactly the opposite of most window shades.

there's the "fog tint". will let light in, but won't let people look in or you out.

check out the "cellular" shades - sort of like a doubled translucent curtain effect - but a window shade.

Top down, bottom up cellular shades are good for this. They also increase home energy efficiency as well by preventing heat loss/gain.

Horizon Window Treatments in Chelsea has a lot of different cellular shade options to choose from.

http://www.horizonyc.com/

frosted glass spray works pretty well

I guess there is no economical solution for windows that would allow me to look outside, but not let outsiders look in?

Plantation shutters. Love them. You can get 2 tiered.. top and bottom so you can let the light in from the upper section when you like.

Any window tint or film works because of the reflection caused by light levels. In the day time, window tints or reflectors (which you can buy as a DYI at HD or Lowes, or hire someone to put on your windows) will reflect the light back and make it difficult for passers-by to see in. If they nosed up right to your window and blocked the light, they would see in. But the opposite is true at night, or even in very overcast days. The light from inside will make the interior fully visible, and truly bright interior lighting shining directly on the windows might even obscure the view out the window.

As others have said, top down shades, shutters, shoji screens or, if you like that sort of thing, stained or patterned glass panels are probably more practical. If you have some serious dollars to disburse, you can get LCD panes that blackout at the touch of a button.

There are shades which are perforated with lots of "microdot" sort of holes which some people have found to work well, but I understand it varies from installation to installation.