Wrt54g

To get pass the L.O.S. problem, see if there is a way to mount it on the roof. I mean, wire length may be a problem, and so could weather, but some more wire and an umbrella should do it!

Obstacles are allways a great problem. While 200 yard or even 300+ are no problem with a free "line of sight", 2,4 GHz WLAN will be greatly reflected by metal, absorbed by water (as its microwaves), and other materials too.

If there is no other option such as a relay station or a higher antenna spot (danger from lightning, needs to be grounded and secured) you can only try another antenna- but if the signal is too weak there is not much you can do, even with a grid or dish antenna.

The most simple solution (and cheapest or easiest to build) are "cantennas" (NO pringles can, since they are suboptimal sizewise) and biquads.
Cantennas require a (smooth at the inside) can, a piece of wire, and a low loss Wifi Cable (H155 for example, since others will loose over 2dbm signal per meter!).
The biquad is rather simple too: Bend a copper wire from a electrical house wire, install a reflector behind it (tinfoil, old CD, anything) and again, coax/wifi cable.

http://images.google.com/images?q=cantenna
http://images.google.com/images?q=wifi+biquad

Also, adding a cone shape to the can will increase the signal gain about 3dbi or such:
http://noether.uoregon.edu/~sadofsky/cookie-cantenna.jpg

Also note that such an antenna will have 8-15dbi gain, depending on the model, build quality and cable length.
3dbi is the double signal strength.
In Europe 100mw/20dbm EIRP (not the output power but the directional strength compared to a theoretical 0dbi all-arround antenna) is legal maximum, I do not know about the US regulations, but I think I do remember its higher.
Most routers have an output of 50-70mW allready. With a 2dbi short rubber antenna thats about 100mW allready, so your scraping legal issues here- usualy no one will care unless you cause lots of disturbance and someone reports it.

Anyway, two more things to keep in mind:
-2,4GHz will most likely get reflected or absorbed by obstacles and rain.
-A antenna with more dbi does not make the signal stronger all arround but focus the "beam". You might get a connection to the other station but get weak reception behind the antenna in the house (and need to install a repeater/AP)
-Polarisation (turning the antenna 90 degrees) might help avoid other disturbances, but both antennas must be turned the same way!
-Helix-Antennas do have a good reception when there are many reflections
http://images.google.com/images?q=wifi+helix
-A different channel (+/-3 channels up or down) can decrease connection problems caused by other stations, wireless video transmitters, and other disturbances