Ppl are retweeting this and I just want to add that faxing is THE most annoying thing to do to your representatives. There are websites where you can fax for a small fee and those machines are LOUD and then at the very least they have to deal with all the print outs.
Barring that: letters are also tangible and have to be handled individually, and showing up in person they have to look in your eyes and turn you away. Maximize how annoying you can be, don’t limit it to just one medium.
District offices are generally close and have fewer staff; a semi-organized weekday lunch of likeminded friends could functionally jam one up for a few hours
Just keep in mind (and be prepared for the fact) that they can and will have you arrested if they decline to give you time and tell you to leave and you don't (which they can also involve the building manager to wash their hands of it since district offices are leased spaces).
