If you remember them by company, file them by company. File them by date. File them by event. File them electronically. Giving cards out at inappropriate times.
People remember poor manners much longer than they remember elegant business cards.
At such events, it's better to ask for permission to send a card to someone's place of business. Expecting one card to do it all. Your consistent use of a well-designed business card is one of the most affordable and acceptable ways to market your business. Make sure YOUR card is kept, remembered, and used. Great giveaway, too.