I think tradeshows are great for the people who are attending, but I'm not so sure for the people who are the vendors.  Most of the people that I have talked with who have attended the tradeshows go just to see what is new and what is happening in the real estate community at large.  Some of the vendors that I have talked to express the same sentiment that you do.  People are interested while at the show, but once they get home or back to the office, day to day life and business seems to overtake them and the tradeshow was just a weekend activity. 

The suggestion that I have regarding follow up would be to actually visit the office of the person who dropped his or her card with you...if the location is near enough to you to do that.  It is harder to ignore someone who walks into your office and it may give you an opportunity to talk with the person who is actually in charge of making decisions for the firm.  Pharmaceutical companies do it with doctors offices all of the time.  They drop in, pass out some samples, spend about ten minutes with the doctor if possible and out they go. 

You could drop in to an office, reintroduce yourself, drop off some brochures, business cards, etc. and then indicate that you have another appointment near the area and must get going.  By physically showing up you are no longer just another person from the tradeshow.  You are reinforcing in his or her mind who you are and what your company can provide for him.  We usually remember people who come by and visit us more easily than someone (among many) we saw at a tradeshow for a few minutes.