It's now expected that you prove your citizenship when entering the United States. The days of passing in from Canada without a birth certificate or passport are gone. Belongings are subject to search without a warrant or probable cause since Fourth Amendment protections against unreasonable search and seizure give way to exemptions that permit a "routine search" by Homeland Security.
There's just not much you can do about it. It's an unpleasant cost of traveling out of the country.
But it may be surprising to learn that "the Border" is actually defined as a 100 mile wide swath that covers the entire "external boundary" of the United States. To put it in perspective, that's 2/3 of our citizenry along with the entirety of Maine and 11 other states. In this zone, Homeland Security can stop, question and search anyone without probable cause or a warrant.
I remembered this the other day after reading Glenn Greenwald's column on data seizure at the border. Homeland Security has been seizing and copying data stored on electronic devices (laptops, cell phones, PDA's) from American citizens they detain at border entry points. Many have not have their devices returned and none have been charged with a crime. Can it happen to us in Skowhegan, Portland, or Auburn? I think the answer is outrageously yes. The ACLU filed suit on the practice in September. Let's hope we can change the policy before it gets even worse.
