"For those who are on the waitlist and want to stand out, deans of admissions recommend that students write a letter expressing interest and updating the school on any recent accomplishments. Many colleges say they don't advise spending money on a campus visit, given the rather small chance of being admitted."
I agree! Send progress reports or final grades, or a recent graded paper, in addition to the letter informing the college you wish to remain on the waitlist.
Source: Wall Street Journal 4/12/2007
College Data maintains historical lists for each college's wait list. Look up any college to see how many were placed on a wait list last year and how many were offered admission. Example: Hamilton College. Search for Hamilton, then click on the college's link. Locate the 'Admission' tab for Hamilton, then scroll to the Wait List stats.
links verified 3/2019