I too do not worry about weight so much and on my more modern layout I do run long trains. The only cars I have added weight to are flat cars. I like to run empty flatcars anywhere in a train so metal wheels and a little weight does the job.
The weights mentioned by Jeff are great, and I have stuck them on under flatcars where they cannot be seen. I have also used lead fishing weights, and have used epoxy putty to glue them into underframe beams. I don't have any standard for the weight of the cars, I just do what seems right. I kit bashed some Bachmann flatcars into piggyback flats and put some relatively light New-Ray truck trailers on them. I added the fifth wheel support and the modern brake wheel stand, and removed the truss rods and added modern truck side frames. I did use fishing weights under these cars and they have been very trouble free runners.
Here is a pic of how they turned out lettered for my Missouri Western modern layout. Sorry about the not cropped image!
