Big article in WSJ today about travel vouchers given out last year approaching their expiration date and what to do about that. It's behind a paywall so can't share a link, but here are some highlights of their advice:
Calendar the actual dates (duh). Carefully read the terms on each voucher. "Beg for mercy" from mean airlines. Complain to the US Dept of Transportation if you were forced to take a voucher instead of a refund (this seems to be a big issue with international airlines). "Game the system," meaning pay to extend a voucher or put it towards a shorter trip and get a new voucher for the unused balance.
I would add, if you have a lot at stake, contact consumer reporters in your area at newspapers or TV stations (you know, those 7 On Your Side and I-Team types). They love a juicy big bad company mistreating a poor defenseless customer story. You could also try a national reporter like Christopher Elliott. It's amazing how in these stories when the reporter contacts the airline, hotel, cruise line, etc., the next day they suddenly decide to make the consumer whole purely by coincidence.
Apparently another tactic is to use Twitter to reach different areas of the company and hashtagging your heart out.