Going with a group of friends is a good idea, especially if you've all got class with the professor. One other route is to contact professors in a particular field through whatever student society you're in. Our Social Justice Program arranges "faculty teas" with professors who don't typically teach 1Ls but do provide upper-level or clinical education in a particular area of the law. The advantage of this route is you can utilize student budgets and don't end up paying out of pocket.
Generally speaking, professors like the opportunity to talk with students outside of class... we had a very candid conversation last spring with our Crim professor about tenure tracks, Friendster.com, and whether they would leave if their closest friend and fellow professor at the law school accepted a position at HLS this year (we hope she doesn't, but we also hope the visiting professor turns HLS down and stays in Nashville to continue directing our Regulatory Lw Program.)
Oh... and check w/ Legal Aid Society or other groups that organize fundraisers. Vanderbilt has an auction every year that includes dinners, poker nights, and karaoke trips with certain members of the faculty. You'll be surprised how much some 3Ls looking at biglaw salaries will pay to have a night on the town with their favorite professor from 1L year.