Temporarily turn off partial file sharing when downloading very popular files. However, this will slow and reduce many downlownloads too.

Better yet, figure out your upstream bandwidth ( http://www.broadbandreports.com/stest ) and set your connection speed to "modem" in the LW prefs (tools->options->speed). Then you can set the upstream bandwidth in LW to something that will keep you below the radar of your ISP's limit AND still allow you to share.

The rule of thumb I use is to make sure my upload total is always greater than the download total (tools->statistics->bandwidth). Being an Ultrapeer also helps if you can't share.