This is a common problem, especially if you have a flu and your nose is clogged. What usually does the trick for me is:
- yawning. luckily flights make me tired in some way
- gulping. chewing gum and a bottle of water will be helpful.
A quick google for "ear pressure relief" adds to the list:
- "Inhale, and then gently exhale while holding the nostrils closed and the mouth shut"
- "Suck on candy" [source]
Wikihow has a similar list on tricks that includes recommendations on medicine. Besides what has already been said they recommend:
- Taking antihistamines before the flight
- special filtered earplugs available in pharmacies and airport stores
Also stay awake during the flight so you don't miss the initial part of the descent sleeping which is usually hardly noticed but makes for a good part of pressure difference to build up which then takes time to compensate (we're talking days here).
In a discussion to a somewhat related question it was mentioned that there are business jets which offer sea-level altitude pressure if you are willing and able to splurge on that.